test-histedit: fix instability creation test
The current test does not rewrite anything and therefor does not create any
instability.
We also clean up the repo state after the test. This required the rebase
extension.
$ "$TESTDIR/hghave" pyflakes || exit 80
$ cd "`dirname "$TESTDIR"`"
$ pyflakes mercurial hgext 2>&1 | "$TESTDIR/filterpyflakes.py"
hgext/inotify/linux/__init__.py:*: 'from _inotify import *' used; unable to detect undefined names (glob)