There was a bug where tracked files inside directories that were
inside ignored directories where not being found by status.  To
make that a little clearer, if you have a .gitignore with:

    ignore/

And then have the following files:

    ignore/dir/tracked     <-- actually a tracked file
    ignore/dir/untracked   <-- should be ignored

Then we would show the tracked file as being removed (because
when we got the to contained item "dir/" inside the ignored
directory, we decided it was safe to skip -- bzzt, wrong!).

This update is much more careful about checking that we are
not skipping over any prefix of a tracked item, regardless of
whether it is ignored or not.

As documented in diff.c, this commit does create behavior that
still differs from core git with regards to the handling of
untracked files contained inside ignored directories.  With
libgit2, those files will just not show up in status or diff.
With core git, those files don't show up in status or diff
either *unless* they are explicitly ignored by a .gitignore
pattern in which case they show up as ignored files.

Needless to say, this is a local behavior difference only, so
it should not be important and (to me) the libgit2 behavior
seems more consistent.