Cloning the ghc repository from GitHub doesn't work out of the box. It
requires installing some special url rewrites into ~/.gitconfig. The
build fails mysteriously if you forget. This patch tries to detect when
you cloned from GitHub, and warns you if you didn't set those url
rewrites.

This hopefully lowers to barrier to contribute to GHC by a tiny bit. At
least one /r/haskell user ran into this recently.

Test Plan:
cloned from github, ran ./boot, saw the message. Installed url
rewrites, and ran ./boot again, didn't see the message.

Reviewed by: austin

Differential Revision: https://phabricator.haskell.org/D1230