sync: add option to skip manifest update

The use case is any situation where your manifest does
not exist on server, but where you still want to do
full sync for the projects, without having your
workspace manifest switched to other branch or
forwarded to latest or similar.
This allows syncing to a historical manifest in git log,
that does not have a branch, as well as when integrating
something together that has not been pushed upstream yet.
Changes can also exist locally on a manifest that is
behind head, meaning not requiring rebase to latest.

Tested using:
  $ cd .repo/manifests/
  $ git checkout <any hash 1>
  $ <do local modifications>
  $ repo sync --no-manifest-update
  $ git checkout <any hash 2>
  $ repo sync --no-manifest-update

Change-Id: I0c9773aa8bc5876813a2e7d7fec697abcb2d9e94
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/246445
Tested-by: Fredrik de Groot <fredrik.de.groot@volvocars.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

subcmds/sync.py

@@ -217,6 +217,10 @@ later is required to fix a server side protocol bug.
     p.add_option('-l', '--local-only',
                  dest='local_only', action='store_true',
                  help="only update working tree, don't fetch")
+    p.add_option('--no-manifest-update','--nmu',
+                 dest='mp_update', action='store_false', default='true',
+                 help='use the existing manifest checkout as-is. '
+                      '(do not update to the latest revision)')
     p.add_option('-n', '--network-only',
                  dest='network_only', action='store_true',
                  help="fetch only, don't update working tree")
@@ -907,7 +911,10 @@ later is required to fix a server side protocol bug.
     if opt.repo_upgraded:
       _PostRepoUpgrade(self.manifest, quiet=opt.quiet)
 
-    self._UpdateManifestProject(opt, mp, manifest_name)
+    if not opt.mp_update:
+      print('Skipping update of local manifest project.')
+    else:
+      self._UpdateManifestProject(opt, mp, manifest_name)
 
     if self.gitc_manifest:
       gitc_manifest_projects = self.GetProjects(args,