feat: gs commit pick (experimental)

[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 mode, same as 'git cherry-pick ',
except it also restacks the upstack branches afterwards.

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.

Limitations:

• Does not support cherry-picking merge commits
• Does not support cherry-picking commits that would cause a conflict

Resolves #372

[abhinav]

This change is part of the following stack:

• git: extract splitNull function #774
  • git: add MergeTree method #833
    • git: add analog for 'git write-tree' #834
      • ui: Add commit selection widget #835
        • git/merge-tree: clarify behavior #840
          • git/merge-tree: Parse file conflict information #842
            • Support experimental commands #854
              • experiment: gs commit fixup #836
                • refac: extract autostash support functionality #890
                  • feat: gs commit pick (experimental) #536 ◀

_{Change managed by git-spice.}

[abhinav]

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