Built a `git-recover-repository` command today. So far it only does the
detection and deletion of corrupt objects, and retrieves them from remotes
when possible. No handling yet of missing objects that cannot be recovered
from remotes.

Here's a couple of sample runs where I do bad things to the git
repository and it fixes them:

<pre>
joey@darkstar:~/tmp/git-annex>chmod 644 .git/objects/pack/*
joey@darkstar:~/tmp/git-annex>echo > .git/objects/pack/pack-a1a770c1569ac6e2746f85573adc59477b96ebc5.pack 
joey@darkstar:~/tmp/git-annex>~/src/git-annex/git-recover-repository 
Running git fsck ...
git fsck found a problem but no specific broken objects. Perhaps a corrupt pack file? Unpacking all pack files.
fatal: early EOF
Unpacking objects: 100% (148/148), done.
Unpacking objects: 100% (354/354), done.
Re-running git fsck to see if it finds more problems.
Re-running git fsck to see if it finds more problems.
Initialized empty Git repository in /home/joey/tmp/tmprepo.0/.git/
Trying to recover missing objects from remote origin
Successfully recovered repository!
You should run "git fsck" to make sure, but it looks like
everything was recovered ok.
</pre>

----

<pre>
joey@darkstar:~/tmp/git-annex>chmod 644 .git/objects/00/0800742987b9f9c34caea512b413e627dd718e
joey@darkstar:~/tmp/git-annex>echo > .git/objects/00/0800742987b9f9c34caea512b413e627dd718e
joey@darkstar:~/tmp/git-annex>~/src/git-annex/git-recover-repository 
Running git fsck ...
error: unable to unpack 000800742987b9f9c34caea512b413e627dd718e header
error: inflateEnd: stream consistency error (no message)
error: unable to unpack 000800742987b9f9c34caea512b413e627dd718e header
error: inflateEnd: stream consistency error (no message)
git fsck found 1 broken objects. Unpacking all pack files.
removing 1 corrupt loose objects
Re-running git fsck to see if it finds more problems.
Re-running git fsck to see if it finds more problems.
Initialized empty Git repository in /home/joey/tmp/tmprepo.0/.git/
Trying to recover missing objects from remote origin
Successfully recovered repository!
You should run "git fsck" to make sure, but it looks like
everything was recovered ok.
</pre>

Works great! I need to move this and `git-union-merge` out of the git-annex
source tree sometime.

----

Today's work was sponsored by Francois Marier.
