checkout --track: make up a sensible branch name if '-b' was omitted

What does the user most likely want with this command?

$ git checkout --track origin/next

Exactly.  A branch called 'next', that tracks origin's branch 'next'.
Make it so.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>