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/init.py

@@ -32,6 +32,7 @@ from wrapper import Wrapper
 
 class Init(InteractiveCommand, MirrorSafeCommand):
   COMMON = True
+  MULTI_MANIFEST_SUPPORT = False
   helpSummary = "Initialize a repo client checkout in the current directory"
   helpUsage = """
 %prog [options] [manifest url]
@@ -90,6 +91,17 @@ to update the working directory files.
 
   def _Options(self, p, gitc_init=False):
     Wrapper().InitParser(p, gitc_init=gitc_init)
+    m = p.add_option_group('Multi-manifest')
+    m.add_option('--outer-manifest', action='store_true',
+                 help='operate starting at the outermost manifest')
+    m.add_option('--no-outer-manifest', dest='outer_manifest',
+                 action='store_false', default=None,
+                 help='do not operate on outer manifests')
+    m.add_option('--this-manifest-only', action='store_true', default=None,
+                 help='only operate on this (sub)manifest')
+    m.add_option('--no-this-manifest-only', '--all-manifests',
+                 dest='this_manifest_only', action='store_false',
+                 help='operate on this manifest and its submanifests')
 
   def _RegisteredEnvironmentOptions(self):
     return {'REPO_MANIFEST_URL': 'manifest_url',