Sometimes, you know the revision you want to land but don't remember which branch it's on. Although `arc branch` allows you to discover this information, we could make it simpler by allowing `arc land X` to behave something like:
- Check if there's a branch/bookmark/etc named `X`.
- If not, and `X` looks like `D123`, check if there's an accepted revision with that monogram.
- If there is, and we can figure out which branch it's on, emit a message, pretend the user typed `arc land <that branch>`, and proceed normally.