evolve: support hg continue and hg abort
hg evolve and hg pick both already support hg abort, but neither of them
support hg continue, and hg fixup supports neither hg continue nor hg abort.
To make the UX a little more consistent with Mercurial core, add support for continue and abort to all three commands.
This doesn't yet change the help text to suggest the use of hg continue and
hg abort on Mercurial versions where they're available.
Edited by Luke Granger-Brown