bisect: add test cases to check that "git bisect start" is atomic

bisect: add test cases to check that "git bisect start" is atomic

This patch adds some test cases to check that "git bisect start"
doesn't leave us in a bad state, especially when it fails.

These test cases show that "git bisect start" is not atomic when it
fails and leave some files like .git/BISECT_START, and in some
cases some refs, over.

The test failures should be fixed in latter commits.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>