merge: add `ACTION_KEEP_ABSENT` to represent files we want to keep absent

Authored by pulkit.


merge: add ACTION_KEEP_ABSENT to represent files we want to keep absent

There are files which were deleted/not present in working copy parent but were
present on other side of merge. On merge, we might decide to keep them deleted.
We want to track such cases more closely, rather all kind of cases which results
from some kind of merging logic.

We do have ACTION_KEEP but having a dedicated action for the absent case is
more cleaner.

Initially I named the action as ACTION_KEEP_DELETED but later realized that
file can be not-present because of other reasons than deletion like rename,
hence decided to use more generic name ACTION_KEEP_ABSENT.

Differential Revision: https://phab.mercurial-scm.org/D8974