unpack-trees: populate cache-tree on successful merge

unpack-trees: populate cache-tree on successful merge

When we unpack trees into an existing index, we discard the old
index and replace it with the new, merged index.  Ensure that this
index has its cache-tree populated.  This will make subsequent git
status and commit commands faster.

Signed-off-by: Brian Degenhardt <bmd@bmdhacks.com>
Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>