bisect: when skipping, choose a commit away from a skipped commit

bisect: when skipping, choose a commit away from a skipped commit

To do that a new function "apply_skip_ratio" is added and another
function "managed_skipped" is created to wrap both "filter_skipped"
and the previous one.

In "managed_skipped" we detect when we should choose a commit away
from a skipped one and then we automatically choose a skip ratio
to pass to "apply_skip_ratio".

The ratio is choosen so that it alternates between 1/5, 2/5 and
3/5.

In "apply_skip_ratio", we ignore a given ratio of all the commits
that could be tested.

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