A `gofmt` pre-commit hook that does what you want

gofmt-git-pre-commit

#!/bin/sh

test -z "$GIT_HOOK_NO_GOFMT" || exit 0

function die {
	echo "FATAL: $@" 1>&2
	exit 128
}

files=""
hash gofmt 2>&- || { echo >&2 "gofmt not in PATH."; exit 1; }
IFS='
'
for file in `git diff --cached --name-only --diff-filter=ACM | grep '\.go$'`; do
	output=`git cat-file -p :$file | gofmt -l 2>&1`
	if test $? -ne 0; then
		output=`echo "$output" | sed "s,<standard input>,$file,"`
		syntaxerrors="${list}${output}\n"
	elif test -n "$output"; then
		files+="$file"
	        list="${list}${file}\n"
	fi
done

exitcode=0
if test -n "$syntaxerrors"; then
	echo >&2 "gofmt found syntax errors:"
	printf "$syntaxerrors"
	exitcode=1
fi
if test -n "$list"; then
	echo >&2 "gofmt will format these files: $files"
	for f in "$files"; do
		set -x
		gofmt -w $f || die "gofmt $f failed"
		git add $f || die "git add $f failed"
	done
fi
exit $exitcode