Merge branch 'mh/ref-iterators'

The API to iterate over all the refs (i.e. for_each_ref(), etc.)
has been revamped.

* mh/ref-iterators:
for_each_reflog(): reimplement using iterators
dir_iterator: new API for iterating over a directory tree
for_each_reflog(): don't abort for bad references
do_for_each_ref(): reimplement using reference iteration
refs: introduce an iterator interface
ref_resolves_to_object(): new function
entry_resolves_to_object(): rename function from ref_resolves_to_object()
get_ref_cache(): only create an instance if there is a submodule
remote rm: handle symbolic refs correctly
delete_refs(): add a flags argument
refs: use name "prefix" consistently
do_for_each_ref(): move docstring to the header file
refs: remove unnecessary "extern" keywords

diff --cc Makefile
Simple merge

diff --cc builtin/fetch.c
Simple merge

diff --cc builtin/remote.c
Simple merge

diff --cc refs.h
Simple merge

diff --cc refs/files-backend.c
Simple merge