fixes#25653. Also:
* Fully document all new hooks, methods, and classes in the upgrader.
* Rename 'language' to 'translation' inside the upgrader.
* Improve the readability of the crazy do-while loop in the is_vcs_checkout() method.
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- Add a "Update Translations" stand-alone button to the updates page
- Shift Language feedback to before update process completion action links & limit the verbosity of output (name + success/errors)
- Simplify/combine the language update descriptive string to only include a plugin/theme name
- Properly handle cache clearing after language updates to prevent langs being repeditively updated
- Display a "All items up to date" string when there's nothing to do
- Reduce the 'Connection Information' from a <h2> to a <h3> to remove duplicate h2's and screen icons from update screens
- Fix the Direct filesystem method not being used for Language updates because WP_LANG_DIR doesn't exist (check it's parent for writable instead)
See #18200, #22704
Built from https://develop.svn.wordpress.org/trunk@25806
git-svn-id: http://core.svn.wordpress.org/trunk@25718 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Andrew Nacin
Dion Hulse