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
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).