Fix mismerge at cdb22c4 in builtin-checkout.c

The code to complain when -b is not given but an explicit --track/--no-track
override was given from the command line was unchanged on one branch and
reworked on the other branch.  The merge result incorrectly kept it.

Spotted by Matt McCutchen.

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