This basically supersedes the current "branch set" command. The plan is to turn
"branch set" into an "upsert" command, and deprecate "branch create". (#3584)
Maybe we can also add "branch set --new" flag to only allow creation of new
branches. One reason behind this proposed change is that "set" usually allows
both "creation" and "update". However, we also need a typo-safe version of
"set" to not create new branches by accident.
"jj branch move" is useful when advancing ancestor branches. Let's say you've
added a couple of commits on top of an existing PR branch, you can advance the
branch by "jj branch move --from 'heads(::@- & branches())' --to @-". If this
pattern is super common, maybe we can add --advance flag for short.
One drawback of this change is that "git branch --move" is equivalent to
"jj branch rename". I personally don't find this is confusing, but it's true
that "move" sometimes means "rename".