From: Johannes Schindelin <redacted>

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 <redacted>
---
 merge-ort.c                      |  7 ++++---
 t/t4301-merge-tree-write-tree.sh | 10 ++++++++++
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/merge-ort.c b/merge-ort.c
index 6491070d965..c37fc035f13 100644
--- a/merge-ort.c
+++ b/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);

diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
index 7d0fa74da74..4ea1b74445d 100755
--- a/t/t4301-merge-tree-write-tree.sh
+++ b/t/t4301-merge-tree-write-tree.sh

@@ -950,5 +950,15 @@ test_expect_success '--merge-base with tree OIDs' '
 	git merge-tree --merge-base=side1^^{tree} side1^{tree} side3^{tree} >with-trees &&
 	test_cmp with-commits with-trees
 '