DWIM "git checkout frotz" to "git checkout -b frotz origin/frotz"

DWIM "git checkout frotz" to "git checkout -b frotz origin/frotz"

When 'frotz' is not a valid object name and not a tracked filename,
we used to complain and failed this command.  When there is only
one remote that has 'frotz' as one of its tracking branches, we can
DWIM it as a request to create a local branch 'frotz' forking from
the matching remote tracking branch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>