sdk: show the complete help when called without arguments

This also fixes the issue where the oneline description has become a bit
stale...

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

git-extra/git-sdk.sh

@@ -118,7 +118,8 @@ EOF
 		esac
 		;;
 	*)
-		sdk die "Usage: sdk ( build-git | init <repo> | create-desktop-icon | help )"
+		printf "Usage: sdk <command> [<argument>...]\n\n" >&2 &&
+		sdk help
 		;;
 	esac
 }