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/start.py

@@ -84,7 +84,8 @@ revision specified in the manifest.
         projects = ['.']  # start it in the local project by default
 
     all_projects = self.GetProjects(projects,
-                                    missing_ok=bool(self.gitc_manifest))
+                                    missing_ok=bool(self.gitc_manifest),
+                                    all_manifests=not opt.this_manifest_only)
 
     # This must happen after we find all_projects, since GetProjects may need
     # the local directory, which will disappear once we save the GITC manifest.
@@ -137,6 +138,6 @@ revision specified in the manifest.
 
     if err:
       for p in err:
-        print("error: %s/: cannot start %s" % (p.relpath, nb),
+        print("error: %s/: cannot start %s" % (p.RelPath(local=opt.this_manifest_only), nb),
               file=sys.stderr)
       sys.exit(1)