Don't fetch theme_roots transient on every page load. Avoid it altogether if...

Don't fetch theme_roots transient on every page load.  Avoid it altogether if there is only one theme dir.  fixes #14911

git-svn-id: https://core.svn.wordpress.org/trunk@15641 1a063a9b-81f0-0310-95a4-ce76da25c4cd