Which Blogging Platform?

I’m looking for a new blogging platform without a lot of success. So, here are the criteria:

• Must store posts in a user-accessible directory structure as text files. I’m not going to be stuck again unable to get posts out if I want to migrate somewhere else.
• Must be able to use Markdown as the preferred markup language.
• Must be self-hosted.
• Technical requirements:
  • Any of PHP, Ruby (but I’m stuck at 1.8.7 on the server so no requirements for 1.9.x), Python…Mac OS 10.6.8 is the server version and we’re stuck there for various reasons for the foreseeable future
  • Simple to install - I don’t want to have to spend days installing and then have a heavy management load. So installation should be fully scripted or installer based
  • Git backend would be a big bonus. We’re running Git with Gitolite on the server. It’s magically easy to install and use and I’m a big fan. No hosted Git solutions though please…so no Github or Heroku
• Should provide existing CSS, templates and blog structure. I don’t want to have to fight for days to set up a site that looks OK and has some basic structures such as menus
• Preferably with its own on-board commenting system
• Must be FOSS (Open Source) software

If you know of anything that fits the bill please talk about it in the comments or head over to Twitter and tell me about it…I’ll be eternally grateful.