[ANNOUNCE] git-submodule-ext, git-new-workdir (updates for supermodules), and git-emeld (using git-new-workdir)

Hello,

I have been working on some of the following utilities for working with submodules:

* git-submodule-ext - Experimental extensions to git-submodule.
Includes 'foreach' with --top-level, --post-order, and --constrain
options. (Been using this to prototype patches for git-submodule)
* git-new-workdir - Modified from git/contrib, I've added support for
using this on supermodules and submodules. It does so by changing some
of the config (which I figured did not matter if the new workdir's are
meant to be short-lived)
* git-emeld - Inspired by git-meld / git-diffall, compare editable
versions of your repos, so you can then go back and make commits.
Since it uses git-new-workdir, it can be used for supermodules as
well.

These are available here: https://github.com/eacousineau/util [Kind of generic name, may change later]

I have also put together a small primer on Git submodules: https://github.com/eacousineau/util/blob/master/SUBMODULES.md

Let me know of any suggestions you have for these tools or the primer. I plan to submit these modifications as patches in the future.

Thank you,
- Eric Cousineau