tutorial: talk about user.name early and don't start with commit -a

tutorial: talk about user.name early and don't start with commit -a

Introducing yourself to git early would be a good idea; otherwise
the user may not find the mistake until much later when "git log"
is learned.

Teaching "commit -a" without saying that it is a shortcut for
listing the paths to commit leaves the user puzzled.  Teach the
form with explicit paths first.

Signed-off-by: Junio C Hamano <redacted>