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 rustfmt test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
$ RUSTFMT=$(rustup which --toolchain nightly-2021-11-02 rustfmt)
$ for f in `testrepohg files 'glob:**/*.rs'` ; do
> $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
> done