submodules and branches

Our CM team likes to use both submodules and branches and it doesn't
always work cleanly.  Some examples:

1. The new release on branch "next_release" adds a submodule, but then the
submodule shows up on all branches including the production branch.

2. For next release on branch "next_release" a submodule was renamed by
moving the target directory, but now git submodule update makes two copies.

Git generally does a good job of tracking renames and deletions, so the fact
that submodules leave stuff behind is surprising.