gc: Add repack option

When a repository is partially cloned, no longer needed blobs are never
removed. To reclaim some of disk space, allow user to pass --repack
which affects only repositories with filter=blob:none and if projects
are not shared.

Change-Id: I0608172c9eff82fb8a6b6ef703eb109fedb7a6cc
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/447722


Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Tested-by: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Scott Lee <ddoman@google.com>