bisect: add parameters to "filter_skipped"

bisect: add parameters to "filter_skipped"

because we will need to get more information from this function in
some later patches.

The new "int *count" parameter gives the number of commits left after
the skipped commit have been filtered out.

The new "int *skipped_first" parameter tells us if the first commit
in the list has been skipped. Note that using this parameter also
changes the behavior of the function if the first commit is indeed
skipped. Because we assume that in this case we will want all the
filtered commits, not just the first one, even if "show_all" is not
set.

So using a not NULL "skipped_first" parameter really means that we
plan to choose to test another commit than the first non skipped
one if the first commit in the list is skipped. That in turn means
that, in case the first commit is skipped, we have to return a
fully filtered list.

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