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
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