[FEATURE REQUEST] Branch rebase and commit cherry-pick

[rlizzo]

Is your feature request related to a problem? Please describe.

Implement cherry-pick and rebase like functionality.

Describe the solution you'd like

Though not a high priority, cherry-picking is one of the primitives required to implement a git-like rebase algorithm, which is the real goal.

While we have the fast-forward and 3-way merge/diff primitives in place, we don't have any real concept of a patch-file operation. Figuring out an analogous method to this (as well as a sane way to determine reasonable merge bases) will be the real heart of this problem.

More context at this phenomenal SO post: https://stackoverflow.com/a/39280674/8312905