[ticket/9824] Handle empty commit messages in commit-msg hook.

Git already handles the case of commit message being empty by
aborting the commit and displaying a reasonably helpful message.
If there is no commit message, the hook will exit with success
exit code to let git do its thing.

PHPBB3-9824

git-tools/hooks/commit-msg

@@ -55,6 +55,17 @@ quit()
 	fi
 }
 
+# Check for empty commit message
+if ! grep -qv '^#' "$1"
+then
+	# Commit message is empty (or contains only comments).
+	# Let git handle this.
+	# It will abort with a message like so:
+	#
+	# Aborting commit due to empty commit message.
+	exit 0
+fi
+
 msg=$(grep -nE '.{81,}' "$1");
 
 if [ $? -eq 0 ]