memfilectx: add a size method

This method is needed to have memfilectx behave like the other file
contexts. We can't just inherit this method because each file context has
different behavior: filectx reads from the filelog, and workingfilectx reads
from the disk. Therefore, we define memfilectx to return the size of the data
in memory.

mercurial/context.py

@@ -1609,6 +1609,8 b' class memfilectx(committablefilectx):'
 
     def data(self):
         return self._data
+    def size(self):
+        return len(self.data())
     def flags(self):
         return self._flags
     def isexec(self):