merge-actions: gather all created action into a set This is mostly to demonstrate we can do this before we start adding more specialized set. Differential Revision: https://phab.mercurial-scm.org/D12115

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

convert: use the merge action constant The constant exists, lets use them. Otherwise we cannot make these constant more powerful. Differential Revision: https://phab.mercurial-scm.org/D12113