histedit: tweak edit message to try and guide users to our workflow
histedit predates evolve, so it drops you on an _uncommitted_ version
of the commit you're amending/splitting, which is in contrast to git
which expects you to use git commit --amend (I think - I'm basing
this on internal bug reports). My hope is that this output will guide
users a little more towards the expected workflow.
Differential Revision: https://phab.mercurial-scm.org/D9560