silence broken link warnings with revs->ignore_missing_links

silence broken link warnings with revs->ignore_missing_links

We set revs->ignore_missing_links to instruct the
revision-walking machinery that we know the history graph
may be incomplete. For example, we use it when walking
unreachable but recent objects; we want to add what we can,
but it's OK if the history is incomplete.

However, we still print error messages for the missing
objects, which can be confusing. This is not an error, but
just a normal situation when transitioning from a repository
last pruned by an older git (which can leave broken segments
of history) to a more recent one (where we try to preserve
whole reachable segments).

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