Merge branch 'tb/ls-refs-optim'

The ls-refs protocol operation has been optimized to narrow the
sub-hierarchy of refs/ it walks to produce response.

* tb/ls-refs-optim:
  ls-refs.c: traverse prefixes of disjoint "ref-prefix" sets
  ls-refs.c: initialize 'prefixes' before using it
  refs: expose 'for_each_fullref_in_prefixes'

diff --cc ls-refs.c
Simple merge

diff --cc ref-filter.c
Simple merge

diff --cc refs.c
Simple merge

diff --cc refs.h
Simple merge