WIP: new command: gs commit pick

[abhinav]

Allows cherry-picking commits into the current branch
and restacks the upstack.
Two modes of usage:

gs commit pick <commit>
gs commit pick

In the first, not much different from 'git cherry-pick'.
In latter form, presents a visualization of commits in upstack branches
to allow selecting one.
--from=other can be used to view branches and commits from elsewhere.

TODO:

• --continue/--abort/--skip flags
• handling of conflicts (continuing after)
• Doc website update

Resolves #372

[abhinav]

This change is part of the following stack:

• WIP: new command: gs commit pick #536 ◀

_{Change managed by git-spice.}

[abhinav]

(PR on hold until I decide how I want to handle cherry-pick failures.)