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