Merge branch 'jc/read-tree-df' (early part)

Merge branch 'jc/read-tree-df' (early part)

* 'jc/read-tree-df' (early part):
  Fix switching to a branch with D/F when current branch has file D.
  Fix twoway_merge that passed d/f conflict marker to merged_entry().
  Fix read-tree --prefix=dir/.
  unpack-trees: get rid of *indpos parameter.
  unpack_trees.c: pass unpack_trees_options structure to keep_entry() as well.
  add_cache_entry(): removal of file foo does not conflict with foo/bar