absorb: be more specific when erroring out on merge commit When you have a merge commit checked out and run `hg absorb`, it would tell you abort: no mutable changeset to change That makes it sound like the problem is public commits when isn't really. Let's be more specific in this case. There was already a test case that test this, so that now prints the new message. I added a new test case that shows the old message (when a public commit is checked out). Differential Revision: https://phab.mercurial-scm.org/D6354