cli git push: no-op refactor of message printing

[ilyagr]

This is based on the commit from #3765 and refactors things some. I could also include this in #3765, or we could reject this PR entirely if it doesn't seem to help.

[yuja]

I don't think mixing UI logic like this is useful.

Let's say we had jj-lib function that classifies branch movements (as well as conflicts), it might return

enum BranchPushUpdate {
    Move { old: CommitId, new: CommitId, movement: Forward|Backward|Sideways },
    Add(CommitId),
    Delete(CommitId),
}

I don't know if this data structure is useful, but it might make sense if we have more than one jj <forge> push commands (or backend needs to know the movement types.) OTOH, BranchPushLabel is basically a UI string constructed from BranchPushUpdate.

[ilyagr]

I'll just close this then.

I was wondering whether to use String vs CommitId, but I thought them essentially equivalent; we could always change one to the other if it started to matter. I thought it might be a mild improvement to collect most of the UI logic in fewer places, but it doesn't matter much, and perhaps you have your own ideas of what you want to do with this code.