filectx: add isabsent method This will indicate whether this filectx represents a file that is *not* in a changectx. This will be used by merge and filemerge code to know about when a conflict is a change/delete conflict. While this is kind of hacky, it is the least bad of all the alternatives. Other options considered but rejected include: - isinstance(fctx, ...) -- not very Pythonic, doesn't support duck typing - fctx.size() is None -- the 'size()' call on workingfilectxes causes a disk stat - fctx.filenode() == nullid -- the semantics around filenode are incredibly confusing. In particular, for workingfilectxes, filenode() is always None no matter whether the file is present on disk or in either parent. Having different behavior for None versus nullid in the merge code is just asking for pain. Thanks to Pierre-Yves David for early review feedback here.