merge-actions: make merge action a full featured object
This open the way for having "smarter" value as action, making the usage code
simpler and more flexible.
We have to explicitly use __bytes__ call in a couple of place because Python2…
Differential Revision: https://phab.mercurial-scm.org/D12114
Issue351: mq: qrefresh can create extra revlog entry
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ hg qinit
$ echo b > b
$ hg ci -A -m foo
adding b
$ echo cc > b
$ hg qnew -f foo.diff
$ echo b > b
$ hg qrefresh
$ hg debugindex b
rev linkrev nodeid p1 p2
0 0 1e88685f5dde 000000000000 000000000000