bisect: store good revisions in a "sha1_array"

bisect: store good revisions in a "sha1_array"

This will make it easier to use good revisions for checking merge
bases later.

To simplify the code, a new "sha1_array_push" function is also
introduced.

And while at it we move the earlier part of the code to fill the
argv that is passed to "setup_revisions", so that all this code is
now completely after "read_bisect_refs".

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