superproject: Only trigger enrollment logic when manifest have it.

The current code would check for enrollment status when the user did not
explicitly specify --[no-]use-superproject and do not have a remembered
value in their repo client. However, because superproject only makes
sense for manifests that have one specified, we should skip the
enrollment logic in that case.

Address this by checking manifest.superproject prior to proceeding. This
would avoid showing the greeting message of superproject enrollment
which can be confusing for developers.

Tested:
  For manifest without superproject:
   - repo sync --use-superproject will still show message for
     superproject;
   - repo sync will not show message regardless of enrollment state
  For manifest with superproject:
   - repo sync will show message and perform enrollment if not
     previously enrolled

Bug: https://crbug.com/gerrit/15039
Change-Id: Ic2be9f9d037f0e7cf3446da474a5a0d0e4bd88da
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/319255
Tested-by: Xin Li <delphij@google.com>
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

git_superproject.py

@@ -410,4 +410,6 @@ def UseSuperproject(opt, manifest):
     if client_value is not None:
       return client_value
     else:
+      if not manifest.superproject:
+        return False
       return _UseSuperprojectFromConfiguration()