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.

mercurial/context.py

@@ -747,6 +747,13 b' class basefilectx(object):'
     def islink(self):
         return 'l' in self.flags()
 
+    def isabsent(self):
+        """whether this filectx represents a file not in self._changectx
+
+        This is mainly for merge code to detect change/delete conflicts. This is
+        expected to be True for all subclasses of basectx."""
+        return False
+
     _customcmp = False
     def cmp(self, fctx):
         """compare with other file context