Merge branch 'kn/for-each-ref-skip'

Merge branch 'kn/for-each-ref-skip'

"git for-each-ref" learns "--start-after" option to help
applications that want to page its output.

* kn/for-each-ref-skip:
  ref-cache: set prefix_state when seeking
  for-each-ref: introduce a '--start-after' option
  ref-filter: remove unnecessary else clause
  refs: selectively set prefix in the seek functions
  ref-cache: remove unused function 'find_ref_entry()'
  refs: expose `ref_iterator` via 'refs.h'