Merge branch 'dt/unpack-trees-cache-tree-revalidate'

Merge branch 'dt/unpack-trees-cache-tree-revalidate'

The code to perform multi-tree merges has been taught to repopulate
the cache-tree upon a successful merge into the index, so that
subsequent "diff-index --cached" (hence "status") and "write-tree"
(hence "commit") will go faster.

The same logic in "git checkout" may now be removed, but that is a
separate issue.

* dt/unpack-trees-cache-tree-revalidate:
  unpack-trees: populate cache-tree on successful merge