sync: save any cruft after calling git gc.

This is ENABLED BY DEFAULT due to data corruption potential.  To disable
it, set REPO_BACKUP_OBJECTS=0 in the environment.

While the workspace will grow over time, this provides a recovery path
for an issue where objects are erroneously deleted from the workspace,
resulting in lost work.  Once the root cause is determined, we will be
able to stop saving backups again.

Backups are kept in .git/objects/.repo/pack.bak

Bug: https://crbug.com/gerrit/16247
Change-Id: Ib8b5c9b4bf0dfa9e29606e0f5c881d65996b2a40
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/345114


Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Xin Li <delphij@google.com>