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/diffmanifests.py

@@ -179,6 +179,9 @@ synced and their revisions won't be found.
   def ValidateOptions(self, opt, args):
     if not args or len(args) > 2:
       self.OptionParser.error('missing manifests to diff')
+    if opt.this_manifest_only is False:
+      raise self.OptionParser.error(
+          '`diffmanifest` only supports the current tree')
 
   def Execute(self, opt, args):
     self.out = _Coloring(self.client.globalConfig)