Merge branch 'kn/for-each-ref'

Merge branch 'kn/for-each-ref'

GSoC project to rebuild ref listing by branch and tag based on the
for-each-ref machinery.  This is its first part.

* kn/for-each-ref:
  ref-filter: make 'ref_array_item' use a FLEX_ARRAY for refname
  for-each-ref: introduce filter_refs()
  ref-filter: move code from 'for-each-ref'
  ref-filter: add 'ref-filter.h'
  for-each-ref: rename variables called sort to sorting
  for-each-ref: rename some functions and make them public
  for-each-ref: introduce 'ref_array_clear()'
  for-each-ref: introduce new structures for better organisation
  for-each-ref: rename 'refinfo' to 'ref_array_item'
  for-each-ref: clean up code
  for-each-ref: extract helper functions out of grab_single_ref()