cabal init should create a .gitignore with dist-newstyle in it.

It's quite irritating to initialise new projects and fall in the same trap each time: `dist-newstyle` is (rightfully) caught by git when one adds the newly created files. This trips quite a large proportion of newcomers I mentor. 

A nice thing would be to have git support through a `--vcs=git` flag so that people who don't care about it don't get their folders cluttered.