The "migrate info (--fixup, no .gitattributes)" test was added
to the t/t-migrate-info.sh test suite in commit
4800c5e of PR git-lfs#4501, when we
added --fixup option support to the "git lfs migrate info" command.

As we expect to introduce additional, similar tests in subsequent
commits in this PR, we rename the test to clarify that no fixup
action is expected in this case, and refactor the test's invocation
of the "git lfs migrate info --fixup" command so that it does not
run inside a pipeline.  This latter change ensures that if the
command fails, the test also fails due to the "set -e" shell option.

While this change to avoid the use of a pipeline is not critical for
this test, it will be important for the similar tests we add in
subsequent commits, because they will need to validate that the
"git lfs migrate info --fixup" command does not simply crash and
produce no output on stdout, which, if it occured in a pipeline,
would not be detected.  This is particularly true in these tests
because we expect the command to produce no output on stdout, which
it would not do both on success and if it panics, so we can not
distinguish these cases by examining the number of lines of output.

We then make a copy of this test in the t/t-migrate-fixup.sh test
suite as well, except using the "git lfs migrate import --fixup"
command, as we also expect to add similar tests to this suite in
subsequent commits.  Note that for this test it is critical that
we supply the --yes option to the command, to avoid it waiting
forever for input because it has detected the log file as a new
file in the Git working tree and therefore has prompted for
confirmation to proceed.

Finally, we remove one unneeded "git add" command from the
setup_single_local_branch_tracked_corrupt() test helper function,
which we refactor further in subsequent commits.

Support for Git macro attributes was added in a series of commits
in PR git-lfs#3391, including commit 9d3e52d,
where the Line structure of the "git/gitattr" package was updated
to include a Macro element which would be set non-nil for macro
definition lines in a .gitattributes file file, while the existing
Pattern element would be set non-nil for all other lines.

The "git lfs track" command, among others, was then adjusted to
create a MacroProcessor structure (from the same "git/gitattr" package)
and call its ProcessLines() method to resolve any macro references
and thus convert the "raw" parsed Line structures into a set for
which the Pattern element was always non-nil, and no Macro elements
appeared.

Later, the "git lfs fsck" command gained the ability to process
macro definitions in .gitattributes files, in PR git-lfs#4525.

However, the "git lfs migrate import" command was not adjusted,
specifically in the implementation of its "--fixup" option, which
initializes a Tree structure (also of the "git/gitattr" package) for
the root tree of each commit in a repository's history using the
package's New() function.  This function traverses all the trees
in the hierarchy and finds and parses all the .gitattributes files
in them.  Then, when the command visits each file within the
commit's tree using the Rewrite() method of the Rewriter structure
in the "git/githistory" package, it calls the (*Tree).Applied()
method to match the file's path against any applicable Git attributes,
to see if the file should be treated as a Git LFS object.

This lack of support for macro attributes in the "git lfs migrate
import --fixup" command was then propagated to the "git lfs migrate
info --fixup" command in commit 4800c5e
of PR git-lfs#4501, when the "git lfs migrate info" command was updated to
respect the --fixup option.

As a result, both of these commands (when used with the --fixup option)
would panic if they encountered a .gitattributes file with any macro
definition, as they would call the (*Tree).Applied() method and it
would attempt to access the nil Pattern element of the lines with
non-nil Macro elements.  (Prior to the changes in commit
c374d1f of PR git-lfs#5375 the "git lfs migrate
import --fixup" command would then stall indefinitely, but it now
also exits after the panic condition.)  These problems were reported
in issue git-lfs#5332.

To resolve this problem and avoid similar ones in the future, we
refactor the Line structure into a Line interface, which only provides
a Attrs() method to retrieve a slice of Attr attributes, and no other
methods.  We then also define two additional interfaces, each of which
embeds the Line interface, PatternLine and MacroLine, with corresponding
getter methods for their respective elements.

The ParseLine() function of the "git/gitattr" package now returns a
slice of generic Line types, each of which is either a PatternLine
or a MacroLine, but never both.  Callers like the Applied() method of
the Tree structure therefore need to perform type assertions or
switches to determine which type of Line they are handling, which
ensures they always access the line's data through safe methods.

We then update the Go tests for the "git/gitattr" package as
appropriate, and also add two tests each to the t/t-migrate-fixup.sh
and t/t-migrate-import.sh test suites.  All four of these new
shell tests fail without the changes in this commit.  In particular,
several of these tests make sure to run the "git lfs migrate" commands
outside of any shell pipeline so the test will fail if the command
panics and produces no output, even if no output is the expected
condition for a successful execution of the command.

Support for Git macro attributes was added in a series of commits
in PR git-lfs#3391, including commit 9d3e52d,
where the Line structure of the "git/gitattr" package was updated
to include a Macro element which would be set non-nil for macro
definition lines in a .gitattributes file file, while the existing
Pattern element would be set non-nil for all other lines.

The "git lfs track" command, among others, was then adjusted to
create a MacroProcessor structure (from the same "git/gitattr" package)
and call its ProcessLines() method to resolve any macro references
and thus convert the "raw" parsed Line structures into a set for
which the Pattern element was always non-nil, and no Macro elements
appeared.

Later, the "git lfs fsck" command gained the ability to process
macro definitions in .gitattributes files, in PR git-lfs#4525.

However, the "git lfs migrate import" command was not adjusted,
specifically in the implementation of its "--fixup" option, which
initializes a Tree structure (also of the "git/gitattr" package) for
the root tree of each commit in a repository's history using the
package's New() function.  This function traverses all the trees
in the hierarchy and finds and parses all the .gitattributes files
in them.  Then, when the command visits each file within the
commit's tree using the Rewrite() method of the Rewriter structure
in the "git/githistory" package, it calls the (*Tree).Applied()
method to match the file's path against any applicable Git attributes,
to see if the file should be treated as a Git LFS object.

This lack of support for macro attributes in the "git lfs migrate
import --fixup" command was then propagated to the "git lfs migrate
info --fixup" command in commit 4800c5e
of PR git-lfs#4501, when the "git lfs migrate info" command was updated to
respect the --fixup option.

As a result, both of these commands (when used with the --fixup option)
panic if they encounter a .gitattributes file with any macro
definitions, as they call the (*Tree).Applied() method and it
attempts to access the nil Pattern element of the lines with
non-nil Macro elements.  (Prior to the changes in commit
c374d1f of PR git-lfs#5375 the "git lfs migrate
import --fixup" command would then stall indefinitely, but it now
also exits after the panic condition.)  These problems were reported
in issue git-lfs#5332.

To resolve this problem and avoid similar ones in the future, we
refactor the Line structure into a Line interface, which only provides
an Attrs() method to retrieve a slice of Attr attributes, and no other
methods.  We then also define two additional interfaces, each of which
embeds the Line interface, PatternLine and MacroLine, with corresponding
getter methods for their respective elements.

The ParseLine() function of the "git/gitattr" package now returns a
slice of generic Line types, each of which is either a PatternLine
or a MacroLine, but never both.  Callers like the Applied() method of
the Tree structure therefore need to perform type assertions or
switches to determine which type of Line they are handling, which
ensures they always access the line's data through safe methods.

We then update the Go tests for the "git/gitattr" package as
appropriate, and also add two tests each to the t/t-migrate-fixup.sh
and t/t-migrate-import.sh test suites.  All four of these new
shell tests fail without the changes in this commit.  In particular,
several of these tests make sure to run the "git lfs migrate" commands
outside of any shell pipeline so the test will fail if the command
panics and produces no output, even if no output is the expected
condition for a successful execution of the command.