"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