A way to obtain a set of attributes given a path, efficiently.

Each path we handle needs one lookup there during checkout, thus it must be lazy and per thread.

git seems to implement it as stack which is efficient if similar paths are handled together, and then drops information as it is no longer needed. This seems like the way to go as we never have to optimize for random access.

Tasks

• an Outcome type to handle searches across groups and can deal with macros.
• implement attribute stack lookups in user-defined order
• a mechanism to reject macros in non-root gitattribute files (should be persistent, with leniency) - it's always lenient but logs issues.
• test for and implement differences (compared to excludes) on how paths match there
• fix gix-ignore test failures due to case-sensitivity
• assure we can handle macro attributes
• assure user-defined order is maintained across multiple Search instances
• a test to see what happens if macros are redefined
• leniency to only log on invalid pattern (similar to git, log errors)
• proper gix gitattribute setup (see git implementation)

Next PRs

• gix-worktree integration (fs::Cache) and support for various sources. Needs to ignore macros in dynamically loaded attr files.
• gix repo attribute query/verify with support for overrides
• A test tool to collect gitattribute and maybe gitignore information from real-world git repos along with the replies by git check-attr to have baselines against which to test our implementation.

Differences to .gitignore

• negative patterns are forbidden
• patterns that match a directory do not recursively match paths
  inside that directory (so using the trailing-slash path/ syntax is
  pointless in an attributes file; use path/** instead)
• built-in macro attributes binary which is -text -diff -merge.
• auto-attributes: text/binary (doesn't seem to be a thing - diff might do its own binary check before diffing blobs if the attribute isn't set)
• macro-resolution: what about cycles?