Fix the git dir lookup reading past the discovered path - #582

git_repository_discover fills a git_buf whose asize is the
allocated capacity, not the length of the path it wrote. gitDirForURL:
copied asize bytes, so the NSData it built carried the path plus
whatever happened to follow it in that allocation.

• Build the path with initWithBytes:length:encoding: over the
  buffer's size, dropping the NSData round-trip that existed only
  to hand a C string to stringWithUTF8String:
• Guard on the resulting path rather than on the allocation size.
  libgit2 documents asize as zero when it does not own the buffer,
  so that test passed only because git_repository_discover happens to
  allocate its own
• Add the first tests for PBRepositoryFinder: repository root, nested
  subdirectory, a directory outside any repository, and a non-file URL

Note that swapping asize for size on its own would be wrong. size
excludes the NUL that libgit2 writes at ptr[size], so an NSData of
exactly size bytes has no terminator and stringWithUTF8String:
would scan past the end of it. Hence building the string from the bytes
and a length instead, which does not depend on a terminator at all.

This also unblocks a libgit2 upgrade: git_buf.asize was renamed
reserved in libgit2 1.x, and this line is the only thing in GitX that
fails to compile against it. With gitx/objective-git#98 checked out,
GitX builds and its tests pass against libgit2 1.9.4 once this lands.

Test plan

• xcodebuild test -workspace GitX.xcworkspace -scheme GitX -only-testing:GitXTests passes, 39 tests, up from 35
• The four new tests git init a repository in a temporary directory
  and resolve symlinks on both sides before comparing, since
  NSTemporaryDirectory() is itself a symlink
• Mutation-checked that the new tests have teeth: changing the encoding
  to NSUTF16StringEncoding fails both lookup tests while the two
  negative tests still pass