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

mergeresult: introduce dedicated tuple for no-op actions This will help us in adding more no-op actions in next patch while keeping the code cleaner.

tests: add criss cross merging tests whose behavior need to be fixed Merging two changesets can mark a file as removed post merge. However, in some cases, a user might not want to remove that file and they revert the removal back and commit the merge. All this works perfectly well. However, when we do criss-cross merges with such merge where user explicitly choose to revert the removal with one where another user choose the removal, we does not get any conflict. The intent here is conflicting and merge should result in conflicts. One user merged and want to keep the file while other user merged and want to remove the file. Merging those merges should result in conflicts. This patch adds test cases for these cases. Differential Revision: https://phab.mercurial-scm.org/D8939