test-histedit-edit.t: demonstrate qnew fails during a histedit (issue4366)
This was accidentally fixed by other work, but given that it's been
broken in the past, I'd like to have a test defending us against
regressions in this area, especially as we add more functionality to
histedit.
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh