merge: deprecate accessing update results by index
Now that we have named attributes, let's convert the code base to use
them. We also add deprecation warnings so legacy consumers are aware
of their transgressions.
``stats.unresolvedcount`` is much easier to read than ``stats[3]``,
don't you think?
Differential Revision: https://phab.mercurial-scm.org/D2694
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