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()