context: add is `isinmemory()` to filectx

This will make it easier to skip certain behavior when running an in-memory
merge.

Differential Revision: https://phab.mercurial-scm.org/D1061

mercurial/context.py

@@ -622,6 +622,9 b' class changectx(basectx):'
     def hidden(self):
         return self._rev in repoview.filterrevs(self._repo, 'visible')
 
+    def isinmemory(self):
+        return False
+
     def children(self):
         """return contexts for each child changeset"""
         c = self._repo.changelog.children(self._node)
@@ -1401,6 +1404,9 b' class committablectx(basectx):'
     def extra(self):
         return self._extra
 
+    def isinmemory(self):
+        return False
+
     def tags(self):
         return []
 
@@ -1978,6 +1984,9 b' class overlayworkingctx(workingctx):'
         else:
             return self._wrappedctx[path].data()
 
+    def isinmemory(self):
+        return True
+
     def filedate(self, path):
         if self.isdirty(path):
             return self._cache[path]['date']