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.