move to git

[warner]

We've decided to move to git. As #1226 points out, keeping Tahoe in darcs is an unfortunate barrier to entry. This ticket tracks the process.

We'll use the repo that I've been maintaining through a creaky manually-run bidirectional darcs-to-git bridge, and publish on github. I started publishing it under my personal account (and thus it has a few dozen forks), but I created a new "Organization" account and we should publicize that one as the official repo (on github, at least):

​https://github.com/tahoe-lafs/tahoe-lafs

The bridge only handles linear history, so until we drop darcs completely, I'll be rebasing and linearizing everything that touches master. The official repo will only contain a "master" branch, for now: later we'll figure out a release process, and add the 1.8.3 branch back in (which is currently unrepresented in git).

Steps:

• Set up "tahoe-lafs" Github "Organization" account
• Set up post-commit webhook to update tahoe-lafs.org:/home/source/git/tahoe-lafs.git upon changes
• Add "git" repo/branch to Trac
  • this is currently stuck at some old revision: I'm not sure why, maybe some bug in the trac-git plugin
• Set up git-based buildmaster
• Migrate all slaves to the git-based buildmaster
  • create entries for all slaves
  • port all Builders (add buildbot "categories" to mark Builders as "supported", "performance", "packaging", etc, to simplify web display)
  • port the index.html page
  • shut down old buildmaster, change slaveport of new buildmaster to match the old one: boom, all slaves now connect to the new buildmaster
  • get Git installed on all buildslaves that lack it
• Stop updating darcs repo, freeze it
• Build tool to modify Trac DB [DARCS-REVID] comments to git-based [REVID/git]
• Run revid-mapping tool
• Switch Trac "default branch" to git
• Replace DB comments with [REVID/git] to just [REVID]
• Remove darcs branch from Trac