Generalization patch

Although  revision 2110 refactored the version control subsystem, it left some bits of Subversion specific code in the core.

The attached patch moves those bits down to the subsystem, and opens the door for other versioning system backends.

This has been proposed upstream, see  ticket 2148.

Complete diff against upstream version of Trac

The complete diff against official Trac 0.9 "Vodun" is attached as well. I will renew it now and then. If it seems too old, just ask!