histedit: add test that an invalid editor doesn't bork client state

We had a report of a situation like this borking a user at Google, but
I can't reproduce it in a test. Let's at least backstop the issue with
a test, so we don't accidentally introduce such a bug...

Differential Revision: https://phab.mercurial-scm.org/D8391

tests/test-histedit-non-commute.t

@@ -300,4 +300,7 b' post message fix'
      summary:     Initial commit
   
 
+An invalid editor shouldn't leave the user in a broken state:
+  $ EDITOR=totally-not-a-thing-unsensible-editor-value hg histedit 4
+
   $ cd ..