rev-list: handle missing tree objects properly

Previously, we assumed only blob objects could be missing. This patch
makes rev-list handle missing trees like missing blobs. The --missing=*
and --exclude-promisor-objects flags now work for trees as they already
do for blobs. This is demonstrated in t6112.

Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -125,6 +125,21 @@ struct rev_info {
 			line_level_traverse:1,
 			tree_blobs_in_commit_order:1,
 
+			/*
+			 * Blobs are shown without regard for their existence.
+			 * But not so for trees: unless exclude_promisor_objects
+			 * is set and the tree in question is a promisor object;
+			 * OR ignore_missing_links is set, the revision walker
+			 * dies with a "bad tree object HASH" message when
+			 * encountering a missing tree. For callers that can
+			 * handle missing trees and want them to be filterable
+			 * and showable, set this to true. The revision walker
+			 * will filter and show such a missing tree as usual,
+			 * but will not attempt to recurse into this tree
+			 * object.
+			 */
+			do_not_die_on_missing_tree:1,
+
 			/* for internal use only */
 			exclude_promisor_objects:1;