git-update-dirs: Create and use mysystem() instead of system()

It prints out what's executed, no need to duplicate that info in print()
statements now. Also move all check_sig() into mysystem().

a8ffe3dc-cd08-11e2-89cc-0800274013ad
Add Local/dgdev/uuid

55ad3754-cd1f-11e2-b538-0050569c0019