[PATCH 1 of 2] histedit: dropmissing: abort on empty plan

[timeless]

Mateusz Kwapich wrote:
> We noticed that many users have the intuition of laving the editor empty when

"laving" should be "leaving".

Also, I'm not quite sure we want to suggest strip unless it's enabled
(there's also prune, and rebase).

I'm +1 to the abort, but not quite sure about the wording.

I think at some point we need to add a hook to core which defines a
list of available extensions which offer "Kill" so that things like
histedit can suggest the best enabled one (prune is better than strip,
but strip is better than prune if only strip is enabled)...