It seems that `go get` explodes and craps over your git directory if you set
GIT_DIR, and some of the go tools run `go get` indirectly, so let's jump
through some hoops to fix it.
* Run the checks against the git index (ie, what you're about to commit),
rather than the working copy
* Add the changes made by `go fmt` back to the index, so that they are included
in the commit, as well as the working copy.
