Rename main talk to .md (for "Markdown")

This is so github automatically renders the content when someone visits
https://github.com/meonkeys/git-talk/blob/master/git-talk.md
Makefile: abstract talk name

I've reused this Makefile a couple times. Abstracting the main talk
content filename will make it even easier next time. Hopefully it
doesn't hurt readability too much.