bisect: fix left over "BISECT_START" file when starting with junk rev

bisect: fix left over "BISECT_START" file when starting with junk rev

Before this patch, when using for example:

$ git bisect start <stuff1> <stuff2>

with <stuff1> or <stuff2> that cannot be parsed as a revision, we
could leave a ".git/BISECT_START" file, from a previous
"git bisect start", alone.

This patch makes sure that it does not happen by removing the
"BISECT_START" file in "bisect_clean_state" and then always writing
it again at the end of "bisect_start".

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