Ordering by `post__in` was introduced in [21776], but the code assumed that
`post__in` would be a comma-separated string listing post IDs. When an array
of post IDs was passed to the `post__in` query var, 'orderby=post__in' was
not respected. This changeset changes this behavior by handling
'orderby=post__in' in the same way as most other values of 'orderby',
which ensures that arrays as well as strings can be properly parsed.
The same treatment is given to the similar `post_name__in` and
`post_parent__in` options of 'orderby', so that most query generation for
orderby clauses happens in the same place, instead of in special cases.
A slight change in the resulting SQL (related to the whitespace around
parentheses and commas) necessitates a change to an existing REST API test
that does a string comparison against the SQL query.
Props mgibbs189, kelvink.
Fixes#38034.
git-svn-id: https://develop.svn.wordpress.org/trunk@44452 602fd350-edb4-49c9-b593-d223f7449a82
082c5d1534