Merge branch 'lt/rename' into HEAD

Merge branch 'lt/rename' into HEAD

* lt/rename:
  Do the fuzzy rename detection limits with the exact renames removed
  Fix ugly magic special case in exact rename detection
  Do exact rename detection regardless of rename limits
  Do linear-time/space rename logic for exact renames
  copy vs rename detection: avoid unnecessary O(n*m) loops
  Ref-count the filespecs used by diffcore
  Split out "exact content match" phase of rename detection
  Add 'diffcore.h' to LIB_H