git-bisect: modernize branch shuffling hack

git-bisect: modernize branch shuffling hack

When switching to a new rev, we first made "new-bisect" branch to
point at the chosen commit, attempt to switch to it, and then
finally renamed the new-bisect branch to bisect by hand when
successful.  This is so that we can catch checkout failure (your
local modification may interfere with switching to the chosen
version) without losing information on which commit the next
attempt should be made.

Rewrite it using a more modern form but without breaking the
safety.

Signed-off-by: Junio C Hamano <gitster@pobox.com>