mark_parents_uninteresting(): drop missing object check

mark_parents_uninteresting(): drop missing object check

We allow UNINTERESTING objects in a traversal to be
unavailable. As part of this, mark_parents_uninteresting()
checks whether we have a particular uninteresting parent; if
not, we will mark it as "parsed" so that later code skips
it.

This code is redundant and even a little bit harmful, so
let's drop it.

It's redundant because when our parse_object() call in
add_parents_to_list() fails, we already quietly skip
UNINTERESTING parents. This redundancy is a historical
artifact. The mark_parents_uninteresting() protection is
from 454fbbcde3 (git-rev-list: allow missing objects when
the parent is marked UNINTERESTING, 2005-07-10). Much later,
aeeae1b771 (revision traversal: allow UNINTERESTING objects
to be missing, 2009-01-27) covered more cases by making the
actual parse more gentle.

  As an aside, even if this weren't redundant, it would be
  insufficient. The gentle parsing handles both missing and
  corrupted objects, whereas the has_object_file() check
  we're getting rid of covers only missing ones.

And the code we're dropping is harmful for two reasons:

  1. We spend extra time on the object lookup, even though
     we don't actually need the information at this point
     (and will just repeat that lookup later when we parse
     for the common case that we _do_ have the object).

  2. It "lies" about the commit by setting the parsed flag,
     even though we didn't load any useful data into the
     struct. This shouldn't matter for the UNINTERESTING
     case, but we may later clear our flags and do another
     traversal in the same process. While pretty unlikely,
     it's possible that we could then look at the same
     commit without the UNINTERESTING flag, in which case
     we'd produce the wrong result (we'd think it's a commit
     with no parents, when in fact we should probably die
     due to the missing object).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>