Make it easier to switch between wit and git

[mwachs5]

This is just one item: #199

Other items:

• make it easy to git submodule deinit / init cleanly
• make it easy to push from a wit manifest.json back to equivalent of doing this fun painful recursive process:

cd <subrepo_with_submodules>
git submodule update --init
cd <some_submodule>
git checkout
cd ..
git checkout -b bump_submodules
git add <some_submodule>
git commit -m "bump submodule to match wit-manifest.json"
git push origin bump_submodules
cd ..
wit -C <top_module> update-dep subrepo_with_submodules
cd top_module
git checkout -b bump_submodules
git add -u wit-manifest.json
git commit -m "Bump wit-manifest.json to pick up some_submodule change"
git push origin bump_submodules
cd some_submodule
git submodule -deinit
wit update-pkg top_module
wit update