evolvecmd: move remaining logic to merge commit desc to existing utility fn This was a bit awful because we had a utility function and still we did some of merging outside of that fn. So let's move the remaining parts there.

amend: allow aborting an `amend --patch` by saving an empty file (issue5925) This patch fixes issue5925 by allow aborting a `hg amend --patch` call by saving an empty patch file. This is similar to aborting of `hg amend` when commit message is empty. Test changes demonstrate the fix.