bisect: move argument parsing before state modification.

bisect: move argument parsing before state modification.

Currently 'git bisect start' modifies some state prior to checking
that its arguments are valid.

This change moves argument validation before state modification
with the effect that state modification does not occur
unless argument validations succeeds.

An existing test is changed to check that new bisect state
is not created if arguments are invalid.

A new test is added to check that existing bisect state
is not modified if arguments are invalid.

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>