bisect: remove too much function nesting

bisect: remove too much function nesting

This patch moves some function calls into "bisect_next_exit" so
that functions are nesting less.

The call to "bisect_rev_setup" is moved from "bisect_common" into
"bisect_next_exit" and the call to "read_bisect_refs" from
"bisect_rev_setup" into "bisect_next_exit".

While at it, "rev_argv" is moved into "bisect_rev_setup".

This will make it easier and cleaner to implement checking merge
bases.

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