Return the matched pathspec pattern in `git_pathspec_match_path`
Instead of returning directly the pattern as the return value, I used an
out parameter, because the function also tests if the passed pathspecs
vector is empty. If yes, it considers that the path "matches", but in
that case there is no matched pattern per se.
Move pathspec code in separate files
Diff uses a `git_strarray` of path specs to represent a subset
of all files to be processed. It is useful to be able to reuse
this filtering in other places outside diff, so I've moved it
into a standalone set of utilities.