Currently, this is dependent on whether the automatic updater is enabled (generally). This is likely to be decoupled in a future release.
This refocuses the auto_core_update_send_email filter to specifically be for success/fail emails for auto updates, while the new filter is more general.
see #25603.
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git-svn-id: http://core.svn.wordpress.org/trunk@25794 1a063a9b-81f0-0310-95a4-ce76da25c4cd