Major code reoganisation, splitting up the single big file. The two goals

kept in mind during this are a) to reduce load time for common cases like
cgi and post-commit and b) make the code easier to navigate.

This also modularises RCS support to the extent that it should be possible
to drop in a module for some RCS other than svn, add a switch for it, and
it pretty much just work.

High chance I missed an edge case that breaks something, this is only
barely tested at this point.