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
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..