bisect: use a detached HEAD to bisect

bisect: use a detached HEAD to bisect

When "git bisect" was first written, it was not possible to
checkout a detached HEAD. The detached feature appeared latter.

That's why before this patch the "git bisect" process used a
"bisect" branch to checkout new revisions to be tested (and also
a "new-bisect" one to check if the checkouts could work).

This patch makes "git bisect" checkout revisions to be tested on
a detached HEAD. This simplifies the code a bit.

The tests to check that "git bisect" does not start if a
"bisect" or a "new-bisect" branch exists are removed as they
are not relevant any more.

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