fakemergerecord: take wlock to write the merge state

The merge state is supposed to be covered by the wlock. We fix the test
extensions to comply to that.

tests/fakemergerecord.py

@@ -16,6 +16,7 b' command = cmdutil.command(cmdtable)'
          [('X', 'mandatory', None, 'add a fake mandatory record'),
           ('x', 'advisory', None, 'add a fake advisory record')], '')
 def fakemergerecord(ui, repo, *pats, **opts):
+    with repo.wlock():
     ms = merge.mergestate.read(repo)
     records = ms._makerecords()
     if opts.get('mandatory'):