superproject - don't update the commit ids of projects that have revisionId.

Pinned manifests and release manifest have revisionId set for all
projects. For such projects don't update the commit ids.

Tested the code with the following commands.

$ ./run_tests -v

$ repo_dev sync --use-superproject -j8

$ repo_dev sync -n -c -j32 -m $(pwd)/manifest_7482982.xml

Bug: [google internal] b/191995372
Change-Id: I4681135b1d15f4a63527b6f0356d76ec842485d6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/310582
Reviewed-by: Xin Li <delphij@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>

git_superproject.py

@@ -298,6 +298,9 @@ class Superproject(object):
     path = project.relpath
     if not path:
       return True
+    # Skip the project with revisionId.
+    if project.revisionId:
+      return True
     # Skip the project if it comes from the local manifest.
     return any(s.startswith(LOCAL_MANIFEST_GROUP_PREFIX) for s in project.groups)