Improvements to output of revisions author data.

* Do not output "false" if avatars are disabled. props ocean90.
* Cache the author data separately to cut down on calls to `get_the_author_meta()` and `get_avatar()`. props nacin.

Fixes #24743.

git-svn-id: https://core.svn.wordpress.org/trunk@24708 1a063a9b-81f0-0310-95a4-ce76da25c4cd