zsh_completion: rename _hg_style_opts to _hg_template_opts
--style is a deprecated alias for --template, and doesn't show up in help
without --verbose, so completing it doesn't make much sense.
Let's s/style/template/ this variable to be more consistent with cmdutil.py,
which is helpful to look at in parallel next time someone wants to update the
completion rules.
Differential Revision: https://phab.mercurial-scm.org/D4261
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