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
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2