sync: added --no-use-superproject to disable superproject.

Tested the code with the following commands.

$ ./run_tests -v

$ repo_dev sync -c -j8 --no-use-superproject
Fetching: 100% (1041/1041), done in 1m22.743s

$ repo_dev sync -c -j8 --use-superproject
WARNING: --use-superproject is experimental and not for general use
..

Bug: [google internal] b/187459275
Change-Id: I3f4269df38cd24a21723e8b2be5a1f013e7b5a91
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/305682
Tested-by: Raman Tenneti <rtenneti@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
diff --git a/subcmds/sync.py b/subcmds/sync.py
index 4d95b02..6f5b564 100644
--- a/subcmds/sync.py
+++ b/subcmds/sync.py
@@ -235,6 +235,9 @@
                  help='fetch submodules from server')
     p.add_option('--use-superproject', action='store_true',
                  help='use the manifest superproject to sync projects')
+    p.add_option('--no-use-superproject', action='store_false',
+                 dest='use_superproject',
+                 help='disable use of manifest superprojects')
     p.add_option('--tags',
                  action='store_false',
                  help='fetch tags')
@@ -276,9 +279,10 @@
 
   def _UseSuperproject(self, opt):
     """Returns True if use-superproject option is enabled"""
-    return (opt.use_superproject or
-            self.manifest.manifestProject.config.GetBoolean(
-                'repo.superproject'))
+    if opt.use_superproject is not None:
+      return opt.use_superproject
+    else:
+      return self.manifest.manifestProject.config.GetBoolean('repo.superproject')
 
   def _GetCurrentBranchOnly(self, opt):
     """Returns True if current-branch or use-superproject options are enabled."""