Support using Graphite commands when checking out a branch

[drewwatkins11]

For users who use Graphite and a stacking workflow, the default checkout command results in an untracked state within the stack. An option could be added for adding or checking out branches using Graphite instead of the standard Git command.

This would be configured on setup in the main configuration file.

If this is an issue that is appreciated as part of the main codebase, I have a PR ready to submit.

[schpet]

yes sounds good. i'm not familiar with graphite so hoping it's a pretty standard/common workflow (sounds like it?)

i've just moved over to jujutsu so good timing to start supporting more vcs'

[schpet]

@drewwatkins11 FYI in case you decide to submit the PRs, note that i added some support for alternative vcs–

4c65d69
be2dd51

hopefully this approach is compatible with the graphite changes you mentioned