revision.c: reduce object database queries

In mark_parents_uninteresting(), we check for the existence of an object file to see if we should treat a commit as parsed. The result is to set the "parsed" bit on the commit. Modify the condition to only check has_object_file() if the result would change the parsed bit. When a local branch is different from its upstream ref, "git status" will compute ahead/behind counts. This uses paint_down_to_common() and hits mark_parents_uninteresting(). On a copy of the Linux repo with a local instance of "master" behind the remote branch "origin/master" by ~60,000 commits, we find the performance of "git status" went from 1.42 seconds to 1.32 seconds, for a relative difference of -7.0%. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>