git_status_file now takes an exact path.

I seem to recall that there are circumstances where you can pass a directory name to `git_status_file` where even without wildcards in the pathspec, you can still get multiple results and an ambiguous case. That may just be old invalid memory on my part, but I'm wondering if this error check should be left in, even with the `GIT_STATUS_OPT_DISABLE_PATHSPEC_MATCH` flag involved.