context: use os.lstat instead of os.stat to fetch file size

When file is a link, we want the size of the link itself, not the size
of the file it points to.

mercurial/context.py

@@ -925,7 +925,7 b' class workingfilectx(filectx):'
         return []
 
     def size(self):
-        return os.stat(self._repo.wjoin(self._path)).st_size
+        return os.lstat(self._repo.wjoin(self._path)).st_size
     def date(self):
         t, tz = self._changectx.date()
         try: