More recent versions of git support paths relative to current directory.
But for maximum compatibility we need to use paths relative to the
file's repo's root.
While it is aesthetically nicer to discard the unused output from `git
ls-files`, it isn't necessary. Discarding requires a shell redirection
which exposes us to vim's shell quirks; it's not worth it.
This bug meant that a realtime diff of a buffer which wasn't the current
one -- which happens during GitGutterAll() -- diffed the staged version
of the buffer against the current buffer's contents instead of the
buffer's contents.
This avoids shelling out twice per buffer: once to check whether git
knows about the file and once to perform the diff. Now we simply do
both in one external call.
Profiling showed external calls to git taking ~20ms. This doesn't seem
too bad but it adds up.
