Add has_tag function to determine if a post has a given tag

[Otto42]

As discussed on wp-hackers, this is a patch for a "has_tag" function. New Loop function which takes a tag slug, id, or name (or array of such) and returns true if the post has that tag, false if it doesn't. Also takes a null input and returns true if the post has any tag.

Useful for theme developers to produce different things depending on tags. Compare to "in_category", as this is basically the same thing but for tags.

Patch for trunk attached.

[Otto42]

Add has_tag function.

[ozh]

For reference: ​http://comox.textdrive.com/pipermail/wp-hackers/2008-April/019284.html
For the patch: +1

[Otto42]

Updated function to handle no tag case better

[westi]

(In [8131]) Add new has_tag() template tag. Fixes #6590 props Otto42.