merge-tree: fail with a non-zero exit code on missing tree objects

When `git merge-tree` encounters a missing tree object, it should error
out and not continue quietly as if nothing had happened.

However, as of time of writing, `git merge-tree` _does_ continue, and
then offers the empty tree as result.

Let's fix this.

Signed-off-by: Johannes Schindelin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: dscho

merge-ort.c

@@ -1659,9 +1659,10 @@ static int collect_merge_info(struct merge_options *opt,
 	info.data = opt;
 	info.show_all_errors = 1;
 
-	parse_tree(merge_base);
-	parse_tree(side1);
-	parse_tree(side2);
+	if (parse_tree(merge_base) < 0 ||
+	    parse_tree(side1) < 0 ||
+	    parse_tree(side2) < 0)
+		return -1;
 	init_tree_desc(t + 0, merge_base->buffer, merge_base->size);
 	init_tree_desc(t + 1, side1->buffer, side1->size);
 	init_tree_desc(t + 2, side2->buffer, side2->size);

t/t4301-merge-tree-write-tree.sh

@@ -951,4 +951,15 @@ test_expect_success '--merge-base with tree OIDs' '
 	test_cmp with-commits with-trees
 '
 
+test_expect_success 'error out on missing tree objects' '
+	git init --bare missing-tree.git &&
+	git rev-list side3 >list &&
+	git rev-parse side3^: >>list &&
+	git pack-objects missing-tree.git/objects/pack/side3-tree-is-missing <list &&
+	side3=$(git rev-parse side3) &&
+	test_must_fail git --git-dir=missing-tree.git merge-tree $side3^ $side3 >actual 2>err &&
+	test_grep "Could not read $(git rev-parse $side3:)" err &&
+	test_must_be_empty actual
+'
+
 test_done