bisect: use "$GIT_DIR/BISECT_START" to check if we are bisecting

bisect: use "$GIT_DIR/BISECT_START" to check if we are bisecting

It seems simpler and safer to use the BISECT_START file everywhere
to decide if we are bisecting or not, instead of using it in some
places and BISECT_NAMES in other places.

In commit 6459c7c6786aa9bda0c7a095c9db66c36da0e5f0 (Nov 18 2007,
Bisect: use "$GIT_DIR/BISECT_NAMES" to check if we are bisecting.),
we decided to use BISECT_NAMES but code changed a lot and we now
have to check BISECT_START first in the "bisect_start" function
anyway.

This patch also makes things a little bit safer by creating
the BISECT_START file first and deleting it last, and also by
adding checks in "bisect_clean_state".

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