same page TOC links have wrong URLs (BB-4018)

The same-page TOC links generated for a sub-directory page are incorrectly prefixed with two directory paths.

For example, the "Subdir/Pagename" content:
{{{
<<toc>>
= Main heading =
== Sub heading ==
}}}

Would result in these two links with duplicate directory paths:
{{

{\\ [[Subdir/Subdir/Pagename#!main-heading]]\\ [[Subdir/Subdir/Pagename#!sub-heading]]\\ }}

}