mergestate: introduce a new ACTION_KEEP_NEW `ACTION_KEEP` is overloaded and it's hard to figure out how we end up with this KEEP, what was the state of things. In a previous patch, we introduced `ACTION_KEEP_ABSENT` which represents files which are kept absent in the working directory. There is another special case where we keep the file when it's not present on both ancestor and remote side. We introduce a dedicated action for that. The goal is to use these information to make bid merge smarter. Differential Revision: https://phab.mercurial-scm.org/D9002