Add better ".git" detection and use

This way, packagers can set GIT_DIR appropriately if they'd prefer to not have ".git" inside their working directory.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

hack/make.sh

@@ -53,9 +53,9 @@ DEFAULT_BUNDLES=(
 )
 
 VERSION=$(cat ./VERSION)
-if [ -d .git ] && command -v git &> /dev/null; then
+if command -v git &> /dev/null && git rev-parse &> /dev/null; then
 	GITCOMMIT=$(git rev-parse --short HEAD)
-	if [ -n "$(git status --porcelain)" ]; then
+	if [ -n "$(git status --porcelain --untracked-files=no)" ]; then
 		GITCOMMIT="$GITCOMMIT-dirty"
 	fi
 elif [ "$DOCKER_GITCOMMIT" ]; then