Auto submod updating

Series of commits and commands that will be used by a CI job in order
to keep the submodules of our repository update today. An example job
run would looking something like:

```
git clone ... code
cd code
scc update-submodules $GIT_BRANCH --push submodule-bump
scc pr $GIT_BRANCH

```

Questions:
 * can we safely determine `base` for the `pr` command?

NB: slightly in progress. This description may be updated.