Merge branch 'js/merge-tree-3-trees'

"git merge-tree" has learned that the three trees involved in the 3-way merge only need to be trees, not necessarily commits. * js/merge-tree-3-trees: fill_tree_descriptor(): mark error message for translation cache-tree: avoid an unnecessary check Always check `parse_tree*()`'s return value t4301: verify that merge-tree fails on missing blob objects merge-ort: do check `parse_tree()`'s return value merge-tree: fail with a non-zero exit code on missing tree objects merge-tree: accept 3 trees as arguments