mergestate: update _stateextras instead of reassinging

`merge.applyupdates()` can store extras for a file and _stateextras may not be
empty always. Hence reassigning loses the old values.

We can directly update like this because we switched to using
`collections.defaultdict` for `_stateextras` sometime ago.

mercurial/mergestate.py

@@ -261,7 +261,7 b' class _mergestate_base(object):'
             hex(fco.filenode()),
             fcl.flags(),
         ]
-        self._stateextras[fd] = {b'ancestorlinknode': hex(fca.node())}
+        self._stateextras[fd][b'ancestorlinknode'] = hex(fca.node())
         self._dirty = True
 
     def addpathconflict(self, path, frename, forigin):