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

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