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.

Start at the cabal init command and trace how newly initialized project files are created. Determine how a --vcs=git option should control creation of a .gitignore containing dist-newstyle, and verify that the generated files match the requested behavior.

Written by the indexing model from the issue text.