Fix compatibility with git 1.5.4.3 (Ubuntu Hardy).

Fix compatibility with git 1.5.4.3 (Ubuntu Hardy).

Thanks to Andy Chong for reporting the problem.

Basically it comes down to two things that are missing in that version but
exist in git 1.5.6:

  - git init --bare doesn't work, but git --bare init does.
  - git cat-file --batch doesn't exist in that version.

Unfortunately, the latter problem is pretty serious; bup join is really slow
without it.  I guess it might be time to implement an internal version of
cat-file.