Override the manifest for the entire command

When a manifest file is overridden, remember that and keep using the
override for the remainder of the process.  If we need to revert it,
make the override name evaluate False.

Change-Id: I1eee05fec6988c1ee4a3c751c4b540d5b5d11797
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/335136
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

main.py

@@ -310,7 +310,7 @@ class _Repo(object):
         # (sub)manifest, and then any child submanifests.
         result = cmd.Execute(copts, cargs)
         for submanifest in repo_client.manifest.submanifests.values():
-          spec = submanifest.ToSubmanifestSpec(root=repo_client.outer_client)
+          spec = submanifest.ToSubmanifestSpec()
           gopts.submanifest_path = submanifest.repo_client.path_prefix
           child_argv = argv[:]
           child_argv.append('--no-outer-manifest')