fsck: reduce stack footprint

fsck: reduce stack footprint

The logic to mark all objects that are reachable from tips of refs were
implemented as a set of recursive functions.  In a repository with a deep
enough history, this can easily eat up all the available stack space.

Restructure the code to require less stackspace by using an object array
to keep track of the objects that still need to be processed.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>