gc: loosen unreferenced objects

gc: loosen unreferenced objects

An unreferenced object might appear in a pack.  This could only happen
because it was previously referenced, and then later that reference
was removed.  When we gc, we copy the referenced objects into a new
pack, and delete the old pack.  This would remove the unreferenced
object.  Now we first create a loose object from any unreferenced
object in the doomed pack.  This kicks off the two-week grace period
for that object, after which it will be collected if it's not
referenced.

This matches the behavior of regular git.

Change-Id: I59539aca1d0d83622c41aa9bfbdd72fa868ee9fb
Signed-off-by: David Turner <dturner@twosigma.com>
Signed-off-by: Jonathan Nieder <jrn@google.com>