refs: expose `ref_iterator` via 'refs.h'

refs: expose `ref_iterator` via 'refs.h'

The `ref_iterator` is an internal structure to the 'refs/'
sub-directory, which allows iteration over refs. All reference iteration
is built on top of these iterators.

External clients of the 'refs' subsystem use the various
'refs_for_each...()' functions to iterate over refs. However since these
are wrapper functions, each combination of functionality requires a new
wrapper function. This is not feasible as the functions pile up with the
increase in requirements. Expose the internal reference iterator, so
advanced users can mix and match options as needed.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>