Tighten up indent function (wikitext, 2df3c11)

This wasn't calculating the string end point correctly nor updating the stored length which meant that it was unnecessarily refilling the buffer every time; this is quite counterproductive seeing as the whole point of the reusable buffer was to avoid re-generating the same indentation string over and over.

Signed-off-by: Greg Hurrell <greg@hurrell.net>