Detect when no push is necessary

Fixes #1

git-mirror.sh

@@ -57,6 +57,16 @@ error() {
 	errors[${#errors[*]}]="$1"
 }
 
+ref_exists() {
+	git show-ref --verify --quiet "$1"
+}
+
+is_ref_at_sha() {
+	sha1="$(git show-ref --verify --hash "$1")"
+	sha2="$2"
+	[[ $sha1 == $sha2 ]]
+}
+
 for branchspec in "${branches[@]}"; do
 	set $branchspec
 	leader_sha="$1"
@@ -66,18 +76,22 @@ for branchspec in "${branches[@]}"; do
 		echo
 		echo "Updating $remote/$branch"
 
-		# Prepare state to push
 		follower_ref="refs/remotes/$remote/$branch"
-		if ! git show-ref --verify --quiet "$follower_ref"; then
+
+		if ! ref_exists "$follower_ref"; then
 			echo "Branch doesn't exist yet at remote, creating"
 			git reset --hard "$leader_sha" --
+			git push "$remote" "+HEAD:$branch" || { error "$remote $branch: failed to push"; continue; }
+
+		elif is_ref_at_sha "$follower_ref" "$leader_sha"; then
+			echo "Branch up to date"
+
 		else
 			echo "Branch existed at remote, updating"
 			git reset --hard "$follower_ref" --
 			git merge --ff-only "$leader_sha" || { error "$remote $branch: FF not possible"; continue; }
-		fi
-
 			git push "$remote" "+HEAD:$branch" || { error "$remote $branch: failed to push"; continue; }
+		fi
 
 	done
 done