These short forms are just to simplify visual recognition and to make your life easier. Git doesn’t really truncate anything, internally everything will be handled with the complete value. You can use a partial SHA-1 at your convenience, though:
Git is smart enough to figure out what commit you meant to type if you provide the first few characters, as long as your partial SHA-1 is at least four characters long and unambiguous — that is, only one object in the current repository begins with that partial SHA-1.