Fix submodules not synced for repeated repo

If I check out e.g. multiple branches of the same repo in one
manifest, only the submodules of one checkout are synced.

Fix this by adding the relative path  of the checkout as a key
to the mapping of derived projects, preventing overwrite.

The fix was originally proposed here:
https://issues.gerritcodereview.com/issues/40013218

Bug: 40013218
Change-Id: Ia86518ccf2c0af7bd7e4daf8d703a55b7e10ba52
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/581062
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Josef Malmstrom <Josef.Malmstrom@arm.com>
Tested-by: Josef Malmstrom <Josef.Malmstrom@arm.com>

command.py

@@ -412,7 +412,8 @@ class Command:
             for project in all_projects_list:
                 if submodules_ok or project.sync_s:
                     derived_projects.update(
-                        (p.name, p) for p in project.GetDerivedSubprojects()
+                        (p.RelPath(local=False), p)
+                        for p in project.GetDerivedSubprojects()
                     )
             all_projects_list.extend(derived_projects.values())
             for project in all_projects_list: