Add gitrepo template and use it in ChemDB published post.

Add gitrepo template and use it in ChemDB published post.

The template is based on ikiwiki's doc/templates/gitbranch.mdwn.

This turned out to be more complicated than I expected, mostly due to
the need for the template to both work and be appropriate Markdown.
The way to do this seems to be to use the htmlscrubber plugin to strip
the <TMPL_...> tags from the rendered version.  I'd disabled the
htmlscrubber plugin because it had been stripping out my MathML.  Now
it's re-enabled, but only for template/* posts.  It would be nice if
there was a way to force posts to evaluate themselves as templates
when rendered directly...

I also added a git tag for the gitrepo template to use.