pre-commit hook: avoid avoid setting GIT_DIR when running go

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.

hooks/pre-commit

@@ -2,8 +2,12 @@
 
 set -eu
 
-# make the GIT_DIR and GIT_INDEX_FILE absolute, before we change dir
-export GIT_DIR=$(readlink -f `git rev-parse --git-dir`)
+# make git_dir and GIT_INDEX_FILE absolute, before we change dir
+#
+# (don't actually set GIT_DIR, because it messes up `go get`, and several of
+# the go commands run `go get` indirectly)
+#
+git_dir=$(readlink -f `git rev-parse --git-dir`)
 if [ -n "${GIT_INDEX_FILE:+x}" ]; then
     export GIT_INDEX_FILE=$(readlink -f "$GIT_INDEX_FILE")
 fi
@@ -19,7 +23,7 @@ cd "$tmpdir"
 # get a clean copy of the index (ie, what has been `git add`ed), so that we can
 # run the checks against what we are about to commit, rather than what is in
 # the working copy.
-git checkout-index -a
+git --git-dir="${git_dir}" checkout-index -a
 
 # run our checks
 golint src/...
@@ -28,6 +32,9 @@ go tool vet --shadow ./src
 gocyclo -over 12 src/
 gb test
 
+# we're done with go so can set GIT_DIR
+export GIT_DIR="$git_dir"
+
 # if there are no changes from the index, we are done
 git diff --quiet && exit 0