largefile: adjust the dirstate post update within a `parentchange` context

This is doing an update, so this seems more appropriate. No other change have been made, but the next changeset will :-)

Differential Revision: https://phab.mercurial-scm.org/D11144

hgext/largefiles/overrides.py

@@ -1796,6 +1796,7 b' def mergeupdate(orig, repo, node, branch'
             raise error.ProgrammingError(
                 b'largefiles is not compatible with in-memory merge'
             )
+        with lfdirstate.parentchange():
         result = orig(repo, node, branchmerge, force, *args, **kwargs)
 
         newstandins = lfutil.getstandinsstate(repo)