Better support for git submodules

[hadrielk]

Enhancements for git submodules:

• When checking out a branch, submodules should be updated (i.e., git submodule update)

• There does not appear to be any way to change a submodule's commit other than by doing so in the git CLI, at which point it will then show up in SM as a submodule change that can be staged+committed for the main repo.

[webit-mdowney]

I feel like this is major, not minor. SM does EVERYTHING better, EXCEPT submodules. Ideally all these actions are in the command palette also (as much as possible).

• add new
• remove existing
• open submodule repo in new window
• view list in sidebar

Unless I am missing something, I cannot find these features.