Bisect: use "$GIT_DIR/BISECT_NAMES" to check if we are bisecting.

Bisect: use "$GIT_DIR/BISECT_NAMES" to check if we are bisecting.

Previously we tested if the "$GIT_DIR/refs/bisect" directory
existed, to check if we were bisecting.

Now with packed refs, it is simpler to check if the file
"$GIT_DIR/BISECT_NAMES" exists, as it is already created when
starting bisection and removed when reseting bisection.

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