icasefs: make case-folding collision detection as deletion aware (issue3648) Before this patch, case-folding collision is checked simply between manifests of each merged revisions. So, files may be considered as colliding each other, even though one of them is already deleted on one of merged branches: in such case, merge causes deleting it, so case-folding collision doesn't occur. This patch checks whether both of files colliding each other still remain after merge or not, and ignores collision if at least one of them is deleted by merge. In the case that one of colliding files is deleted on one of merged branches and changed on another, file is considered to still remain after merge, even though it may be deleted by merge, if "deleting" of it is chosen in "manifestmerge()". This avoids fail to merge by case-folding collisions after choices from "changing" and "deleting" of files. This patch adds only tests for "removed remotely" code paths in "_remains()", because other ones are tested by existing tests in "test-casecollision-merge.t".