Code clean-up.

* jk/unused-params:
  ref-filter: drop unused "sz" parameters
  ref-filter: drop unused "obj" parameters
  ref-filter: drop unused buf/sz pairs
  files-backend: drop refs parameter from split_symref_update()
  pack-objects: drop unused parameter from oe_map_new_pack()
  merge-recursive: drop several unused parameters
  diff: drop complete_rewrite parameter from run_external_diff()
  diff: drop unused emit data parameter from sane_truncate_line()
  diff: drop unused color reset parameters
  diff: drop options parameter from diffcore_fix_diff_index()