Add option to correct gitdir when syncing

In some cases, a user may wish to continue with a sync even though
it would require overwriting an existing git directory. This behavior
is not safe as a default because it could result in the loss of some
user data, but as an optional flag it allows the user more flexibility.

To support this, add a --force-sync flag to the sync command that will
attempt to overwrite the existing git dir if it is specified and the
existing git dir points to the wrong obj dir.

Change-Id: Ieddda8ad54e264a1eb4a9d54881dd6ebc8a03833

subcmds/sync.py

@@ -119,6 +119,11 @@ credentials.
 The -f/--force-broken option can be used to proceed with syncing
 other projects if a project sync fails.
 
+The --force-sync option can be used to overwrite existing git
+directories if they have previously been linked to a different
+object direcotry. WARNING: This may cause data to be lost since
+refs may be removed when overwriting.
+
 The --no-clone-bundle option disables any attempt to use
 $URL/clone.bundle to bootstrap a new Git repository from a
 resumeable bundle file on a content delivery network. This
@@ -174,6 +179,11 @@ later is required to fix a server side protocol bug.
     p.add_option('-f', '--force-broken',
                  dest='force_broken', action='store_true',
                  help="continue sync even if a project fails to sync")
+    p.add_option('--force-sync',
+                 dest='force_sync', action='store_true',
+                 help="overwrite an existing git directory if it needs to "
+                      "point to a different object directory. WARNING: this "
+                      "may cause loss of data")
     p.add_option('-l', '--local-only',
                  dest='local_only', action='store_true',
                  help="only update working tree, don't fetch")
@@ -281,6 +291,7 @@ later is required to fix a server side protocol bug.
         success = project.Sync_NetworkHalf(
           quiet=opt.quiet,
           current_branch_only=opt.current_branch_only,
+          force_sync=opt.force_sync,
           clone_bundle=not opt.no_clone_bundle,
           no_tags=opt.no_tags, archive=self.manifest.IsArchive,
           optimized_fetch=opt.optimized_fetch)
@@ -696,7 +707,7 @@ later is required to fix a server side protocol bug.
     for project in all_projects:
       pm.update()
       if project.worktree:
-        project.Sync_LocalHalf(syncbuf)
+        project.Sync_LocalHalf(syncbuf, force_sync=opt.force_sync)
     pm.end()
     print(file=sys.stderr)
     if not syncbuf.Finish():