githelp: clean up reflog help
This referenced commands that don't exist in core. The new help
isn't great since it references an experimental extension. But
it is better than nothing.
While we're here, also add test coverage.
Differential Revision: https://phab.mercurial-scm.org/D1727
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors