merge: honor prepare-commit-msg hook

When a merge is stopped due to conflicts or --no-commit, the
subsequent commit calls the prepare-commit-msg hook. However,
it is not called after a clean merge. Fix this inconsistency
by invoking the hook after clean merges as well.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t7505-prepare-commit-msg-hook.sh b/t/t7505-prepare-commit-msg-hook.sh
index ff18962..5b4b694 100755
--- a/t/t7505-prepare-commit-msg-hook.sh
+++ b/t/t7505-prepare-commit-msg-hook.sh
@@ -132,6 +132,18 @@
 
 '
 
+test_expect_success 'with hook (merge)' '
+
+	head=`git rev-parse HEAD` &&
+	git checkout -b other HEAD@{1} &&
+	echo "more" >> file &&
+	git add file &&
+	git commit -m other &&
+	git checkout - &&
+	git merge other &&
+	test "`git log -1 --pretty=format:%s`" = merge
+'
+
 cat > "$HOOK" <<'EOF'
 #!/bin/sh
 exit 1