I have found this the most reliable way to recover from a corrupt git repository. I have had a lot of them lately, there might be a regression in btrfs in Ubuntu's Linux 3.8.0-33 (!).

  1. Create a clone of a known good repository.
  2. Add the clone as an object alternate to the broken repository.
  3. Do a git-repack -a -d to lift the external objects into repo-local packs.
  4. Remove the clone
$ cd /tmp/
$ git clone good-host:/path/to/good-repo
$ cd /home/user/broken-repo
$ echo /tmp/good-repo/.git/objects/ > .git/objects/info/alternates
$ git repack -a -d
$ rm -rf /tmp/good-repo

... and push early, push often. ;-)