Fix post-merge.sh exit status in some cases

This commit tweaks the function runOnChange() to make sure it always
returns the correct exit status. Before this change, this function would
return 1 instead of 0 when there was no change to composer.lock and
package-lock.json in the commits that were merged. This resulted in
husky mistakenly reporting a failure when actually the post-merge script
succeeded.

bin/post-merge.sh

@@ -3,7 +3,10 @@
 changedFiles="$(git diff-tree -r --name-only --no-commit-id ORIG_HEAD HEAD)"
 
 runOnChange() {
-	echo "$changedFiles" | grep -q "$1" && eval "$2"
+	if echo "$changedFiles" | grep -q "$1"
+	then
+		eval "$2"
+	fi
 }
 
 runOnChange "package-lock.json" "npm install"