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/gitc_init.py

@@ -24,6 +24,7 @@ import wrapper
 
 class GitcInit(init.Init, GitcAvailableCommand):
   COMMON = True
+  MULTI_MANIFEST_SUPPORT = False
   helpSummary = "Initialize a GITC Client."
   helpUsage = """
 %prog [options] [client name]