merge-actions: add an explicite "no_op" attribute
This make the MergeAction smarter and able to describe themself. This is useful
to help introducing more MergeAction object that better the complexity of the
situation.
Differential Revision: https://phab.mercurial-scm.org/D12116
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors