FR: swap command (was known as "better tools for reordering commits part 1")

[ilyagr]

A swap command could have the obvious UI: jj swap rev1 rev2. I'm not sure if we can do better than:

• Both revsets have to resolve to single revisions
• The parents of rev1 will be whatever the parents of rev2 were. This behavior would be a bit confusing for empty merge commits (which are common), so I'd disallow merge commits by default and have an option to enable it.

In terms of implementation, this is related to #1188.

This is a baby version of #1531.

[dbarnett]

Instead of creating two different command names, could one command do both if you support a non-interactive variant of reorder under the same command name? Initially you could support whatever special case operations are easiest and reject attempts to do anything else.

Personally if I had a full reorder command I'd just forget swap every existed and consider it clutter.

[ilyagr]

Yes, the more I think of it the less attractive it becomes. What you really want, usually, is to move a commit to before/after another commit. Martin pointed that out in #1188 (comment).