list-objects.c: don't add an unparsed NULL as a pending tree

"git rev-list --first-parent --boundary $commit^..$commit" segfaults on a
merge commit since 8d2dfc4 (process_{tree,blob}: show objects without
buffering, 2009-04-10), as it tried to dereference a commit that was
discarded as UNINTERESTING without being parsed (hence lacking "tree").

Signed-off-by: Junio C Hamano <gitster@pobox.com>

list-objects.c

@@ -147,7 +147,12 @@ void traverse_commit_list(struct rev_info *revs,
 	struct commit *commit;
 
 	while ((commit = get_revision(revs)) != NULL) {
-		add_pending_tree(revs, commit->tree);
+		/*
+		 * an uninteresting boundary commit may not have its tree
+		 * parsed yet, but we are not going to show them anyway
+		 */
+		if (commit->tree)
+			add_pending_tree(revs, commit->tree);
 		show_commit(commit);
 	}
 	for (i = 0; i < revs->pending.nr; i++) {

t/t6110-rev-list-sparse.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+test_description='operations that cull histories in unusual ways'
+. ./test-lib.sh
+
+test_commit () {
+	echo "$1" >"$1.file" &&
+	git add "$1.file" &&
+	test_tick &&
+	git commit -m "$1"
+}
+
+test_expect_success setup '
+	test_commit A &&
+	test_commit B &&
+	test_commit C &&
+	git checkout -b side HEAD^ &&
+	test_commit D &&
+	test_commit E &&
+	git merge master
+'
+
+test_expect_success 'rev-list --first-parent --boundary' '
+	git rev-list --first-parent --boundary HEAD^..
+'
+
+test_done