superproject: add projects from local manifest to local::<filename> group.

With repo sync --use-superproject, don't update the commit ids of every project
that comes from local manifest.

Tested the code with the following commands.

$ ./run_tests -v

+ Test with local.xml

1. repo init --use-superproject -u persistent-https://googleplex-android.git.corp.google.com/a/platform/manifest

2. cd .repo
cp -r /google/src/head/depot/google3/wireless/android/build_tools/translations/pipeline/local_manifests local_manifests
cd ..

local$ time repo_dev sync --use-superproject
NOTICE: --use-superproject is in beta; report any issues to the address described in `repo version`
.../local/.repo/exp-superproject/feb2c2847da5e274f3d530d5ab438af8-superproject.git: Initial setup for superproject completed.
...

Bug: [google internal] b/189360443
Bug: [google internal] b/189139268
Bug: https://crbug.com/gerrit/14499
Change-Id: Ideaf268c294e9b500b2b9726ffbd733dd8d63004
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/308822
Tested-by: Raman Tenneti <rtenneti@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>

manifest_xml.py

@@ -34,6 +34,9 @@ MANIFEST_FILE_NAME = 'manifest.xml'
 LOCAL_MANIFEST_NAME = 'local_manifest.xml'
 LOCAL_MANIFESTS_DIR_NAME = 'local_manifests'
 
+# Add all projects from local manifest into a group.
+LOCAL_MANIFEST_GROUP_PREFIX = 'local:'
+
 # ContactInfo has the self-registered bug url, supplied by the manifest authors.
 ContactInfo = collections.namedtuple('ContactInfo', 'bugurl')
 
@@ -679,7 +682,9 @@ https://gerrit.googlesource.com/git-repo/+/HEAD/docs/manifest-format.md
               # Since local manifests are entirely managed by the user, allow
               # them to point anywhere the user wants.
               nodes.append(self._ParseManifestXml(
-                  local, self.repodir, restrict_includes=False))
+                  local, self.repodir,
+                  parent_groups=f'{LOCAL_MANIFEST_GROUP_PREFIX}:{local_file[:-4]}',
+                  restrict_includes=False))
         except OSError:
           pass