Update submodules on git checkout

[mfine]

When checking out a git repository, also pull in the submodules. The changes here call git submodule update --init --recursive once the repo is cloned, and then if there's a hash to checkout, to call git submodule update --recursive after the hash has been checked out.

Not sure what best practices are around git submodules are - both of these commands seemingly succeed on repos without submodules - but they could be alternatively conditionally run if a .gitmodules file was present?

/cc @nzoschke

Release Playbook

• Rebase against master
• Release branch ()
• Pass CI ()
• Code review
• Merge into master
• Release master ()
• Pass CI ()
• Update staging rack
• Edit Rack release record and/or update docs
• Publish release
• Release CLI

[ddollar]

Closes #729

[MiguelMoll]

Thanks! I'll be merging this into #739 for a release.