Unserialize `get_metadata()` results when 'key' is omitted.

Props mattkeys, nacin.
Fixes #15030.
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Twenty Fifteen: Change version compare string to '4.1-alpha' to avoid an error for alpha and beta testers during development.

fixes #30137.
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Use correct file name in a hook comment.

props wedi.
fixes #30200.
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Move `term_exists()` tests to their own file.
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Correct two parameter types in the `wp_save_post_revision_check_for_changes` hook documentation.

Props johnbillion.
Fixes #30185.

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