Merge branch 'lt/unpack-trees'

Merge branch 'lt/unpack-trees'

* lt/unpack-trees:
  unpack_trees(): fix diff-index regression.
  traverse_trees_recursive(): propagate merge errors up
  unpack_trees(): minor memory leak fix in unused destination index
  Make 'unpack_trees()' have a separate source and destination index
  Make 'unpack_trees()' take the index to work on as an argument
  Add 'const' where appropriate to index handling functions
  Fix tree-walking compare_entry() in the presense of --prefix
  Move 'unpack_trees()' over to 'traverse_trees()' interface
  Make 'traverse_trees()' traverse conflicting DF entries in parallel
  Add return value to 'traverse_tree()' callback
  Make 'traverse_tree()' use linked structure rather than 'const char *base'
  Add 'df_name_compare()' helper function