git option in config should be an enumeration

git option in config should be an enumeration

The option in ~/.cargo/config is now called `vcs` and has the following
options: "git", "hg", and "none". This corresponds to the options for
the --vcs argument to `cargo new`.

This also fixes detection for hg repositories. Previously, we only tried
to discover if the working directory was contained in a git repository.
If it was, we would skip creating a git repo. Now, we check if the
working directory is contained in either a git or mecurial repo. If it
is, we skip creating a new repo.

Fixes #1116