Merge branch 'jd/unpack-trees-wo-the-repository' into jch

A handful of inappropriate uses of the_repository have been
rewritten to use the right repository structure instance in the
unpack-trees.c codepath.

Comments?

* jd/unpack-trees-wo-the-repository:
  unpack-trees: use repository from index instead of global
  unpack-trees: use repository from index instead of global