rev-list: add list-objects filtering support

rev-list: add list-objects filtering support

Teach rev-list to use the filtering provided by the
traverse_commit_list_filtered() interface to omit
unwanted objects from the result.

Object filtering is only allowed when one of the "--objects*"
options are used.

When the "--filter-print-omitted" option is used, the omitted
objects are printed at the end.  These are marked with a "~".
This option can be combined with "--quiet" to get a list of
just the omitted objects.

Add t6112 test.

In the future, we will introduce a "partial clone" mechanism
wherein an object in a repo, obtained from a remote, may
reference a missing object that can be dynamically fetched from
that remote once needed.  This "partial clone" mechanism will
have a way, sometimes slow, of determining if a missing link
is one of the links expected to be produced by this mechanism.

This patch introduces handling of missing objects to help
debugging and development of the "partial clone" mechanism,
and once the mechanism is implemented, for a power user to
perform operations that are missing-object aware without
incurring the cost of checking if a missing link is expected.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>