[Trac] preliminary darcs implementation

[Justus Pendleton]

I have a first pass at letting trac talk to darcs repositories.  It isn't
complete but seems to work for what I've done.  I've added infrastructure so
that other systems -- monotone, arch -- could conceivably be used as well, but I
haven't bothered with the backends for those.  Having trac use multiple
repositories shouldn't be too hard, either.

For those who want to take a look, the darcs repository of my changes is
available at http://www.ryoohki.net/~justus/trac and I have a tracd running at
http://www.ryoohki.net:8080 against a darcs repository.

What works:
- the Timeline view
- the Changeset view, although I haven't added in diffs yet
- the Browser view, but only for "head" and it is pretty slow right now
- tracd
- all of the stuff that didn't talk to subversion directly: Wiki, Tickets, and
Roadmap
- subversion access still works

What doesn't work:
- trac-admin.  to create a darcs repository you need to set up an env by hand

What I've done is create a new package trac.repository that has a factory
method.  Each backend has its own class and needs to support the API that is
loosely documented in Repository.py.  So I ripped out all of the svn specific
stuff and put it in trac/repository/Subversion.py, trying to clean up the
interfaces a little bit but generally leaving the code as it was.

There is also a new configuration option "repository" which has the format
repository = TYPE LOCATION
repository = svn /var/www/svn

If you don't have repository defined but do have the old repos_dir config
variable then it uses that path and assumes it is a subversion repository.

I'll be working on adding the rest of the darcs support and possibly cleaning up
the code and interfaces in the near future.  I also need to test everything lots
to make sure a) darcs works more or less as expected and, b) subversion still
works as it used to.

Happy New Year,
Justus