t4301: verify that merge-tree fails on missing blob objects

We just fixed a problem where `merge-tree` would not fail on missing
tree objects. Let's ensure that that problem does not occur with blob
objects (and won't, in the future, either).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

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

@@ -961,4 +961,18 @@ test_expect_success 'error out on missing tree objects' '
 	test_must_be_empty actual
 '
 
+test_expect_success 'error out on missing blob objects' '
+	seq1=$(test_seq 1 10 | git hash-object -w --stdin) &&
+	seq2=$(test_seq 1 11 | git hash-object -w --stdin) &&
+	seq3=$(test_seq 0 10 | git hash-object -w --stdin) &&
+	tree1=$(printf "100644 blob %s\tsequence" $seq1 | git mktree) &&
+	tree2=$(printf "100644 blob %s\tsequence" $seq2 | git mktree) &&
+	tree3=$(printf "100644 blob %s\tsequence" $seq3 | git mktree) &&
+	git init --bare missing-blob.git &&
+	test_write_lines $seq1 $seq3 $tree1 $tree2 $tree3 |
+	git pack-objects missing-blob.git/objects/pack/side1-whatever-is-missing &&
+	test_must_fail git --git-dir=missing-blob.git merge-tree --merge-base=$tree1 $tree2 $tree3 >actual &&
+	test_must_be_empty actual
+'
+
 test_done