git-svn: proper detection of bare repositories

git-svn: proper detection of bare repositories

When in a bare repository (or .git, for that matter), git-svn would fail
to initialise properly, since git rev-parse --show-cdup would not output
anything.  However, git rev-parse --show-cdup actually returns an error
code if it's really not in a git directory.

Fix the issue by checking for an explicit error from git rev-parse, and
setting $git_dir appropriately if instead it just does not output.

Signed-off-by: Deskin Miller <deskinm@umich.edu>
Acked-by: Eric Wong <normalperson@yhbt.net>