ref-filter: make internal reachable-filter API more precise

ref-filter: make internal reachable-filter API more precise

The internal reachable-filter API is a bit loose and imprecise; it
also bleeds unnecessarily into the public header. Tighten the API
by:

* renaming do_merge_filter() to reach_filter()

* separating parameters to explicitly identify what data is used
  by the function instead of passing an entire ref_filter_cbdata
  struct

* renaming and moving internal constants from header to source
  file

Signed-off-by: Aaron Lipman <alipman88@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>