Add multi-manifest support with <submanifest> element

To be addressed in another change:
 - a partial `repo sync` (with a list of projects/paths to sync)
   requires `--this-tree-only`.

Change-Id: I6c7400bf001540e9d7694fa70934f8f204cb5f57
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/322657
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

subcmds/diff.py

@@ -50,7 +50,7 @@ to the Unix 'patch' command.
     return (ret, buf.getvalue())
 
   def Execute(self, opt, args):
-    all_projects = self.GetProjects(args)
+    all_projects = self.GetProjects(args, all_manifests=not opt.this_manifest_only)
 
     def _ProcessResults(_pool, _output, results):
       ret = 0