In 8ae9540f2c, I made `jj move/squash/unsquash` not abandon the
working copy if it became empty because that would lose any
description associated with it. It turned out that the new behavior
was also confusing because it made it unclear if the working-copy
commit was actually abandoned. Let's roll back that change and instead
ask the user for a combined description when both the source and
destination commits have non-empty descriptions. Not discarding a
non-empty description seems like a good improvement regardless of the
behavior related to working-copy commits. It's also how `hg fold`
behaves (though hg doesn't allow the description to be empty).
With this patch, the order is this:
`$JJ_EDITOR` environement variable
`ui.editor` config
`$VISUAL` environement variable
`$EDITOR` environement variable
`pico`
That matches git, except that git falls back to an editor determined
at compile time (usually `vi`) instead of using `pico`.
