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()