repo info findRemoteLocalDiff use short branch

When running repo info -d an error would be thrown saying:
  fatal: bad revision 'refs/remotes/m/refs/heads/master..'

Using the short branch name here instead, like 'refs/remotes/m/master..'
resolves this issue.

Signed-off-by: Daniel Kutik <daniel.kutik@lavawerk.com>
Change-Id: I50ea92c45c011b2c3e3a63803decb88e7837a380
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/266578
Reviewed-by: Mike Frysinger <vapier@google.com>