D5477: branches: add -r option to show branch name(s) of a given rev (issue5948)

[Yuya Nishihara]

>   We like the UX of `hg branch --rev R XXX` for changing the branch of a commit, it's the same UX than bookmark and topic. It's also easier than `hg branch --force XXX; hg amend`.

What I don't like about `hg branch --rev` is that it actually rewrites
the commits, which is IMHO different in UX from `hg branch` and `hg bookmark`.

It's also different in that both `hg branch` (with no rev) and `hg bookmark [-r]`
specify a branch head (suppose the working change will eventually be new
branch head), whereas `hg branch -r REV` specifies a set of revisions to be
re-branched.

(Maybe we should move the discussion to the list if it continues. I hate Phab
emails.)