merge: don't call update hook when using in-memory context

I'm pretty sure many hook implementors will assume that they can
inspect the working copy and/or dirstate parents when the hook is
called, so I don't think we should call the hook when using an
in-memory context. The new behavior matches that of the preupdate
hook.

Differential Revision: https://phab.mercurial-scm.org/D7898

mercurial/merge.py

@@ -2574,7 +2574,7 b' def update('
     if not branchmerge:
         sparse.prunetemporaryincludes(repo)
 
-    if not partial:
+    if updatedirstate:
         repo.hook(
             b'update', parent1=xp1, parent2=xp2, error=stats.unresolvedcount
         )