manifest_xml: group for submanifest projects

Add all projects in a submanifest to the group
submanifest::<path_prefix> for ease in filtering.

Change-Id: Ia6f01f9445f4f8d20fda3402f4d5821c43ceaf7f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/331319
Tested-by: LaMont Jones <lamontjones@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

docs/manifest-format.md

@@ -281,6 +281,9 @@ with the new settings needed.
 If not supplied the remote and project for this manifest will be used: `remote`
 cannot be supplied.
 
+Projects from a submanifest and its submanifests are added to the
+submanifest::path:<path_prefix> group.
+
 Attribute `manifest-name`: The manifest filename in the manifest project.  If
 not supplied, `default.xml` is used.