Merge branch 'en/unpack-trees-split-index-fix'

The split-index feature had a long-standing and dormant bug in
certain use of the in-core merge machinery, which has been fixed.

* en/unpack-trees-split-index-fix:
unpack_trees: fix breakage when o->src_index != o->dst_index

diff --cc unpack-trees.c
Simple merge