basefilectx: move isexec and islink from memfilectx

This will be used in the future for creating memctx objects from other
store-type objects, such as a patch store or even another context.

mercurial/context.py

@@ -714,6 +714,10 b' class basefilectx(object):'
             return util.binary(self.data())
         except IOError:
             return False
+    def isexec(self):
+        return 'x' in self.flags()
+    def islink(self):
+        return 'l' in self.flags()
 
     def cmp(self, fctx):
         """compare with other file context
@@ -1656,9 +1660,5 b' class memfilectx(committablefilectx):'
         return len(self.data())
     def flags(self):
         return self._flags
-    def isexec(self):
-        return 'x' in self._flags
-    def islink(self):
-        return 'l' in self._flags
     def renamed(self):
         return self._copied