refs.c: add reflog backend methods

Add methods for the reflog functions.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: add methods for misc ref operations

Add ref backend methods for:
resolve_ref_unsafe, is_refname_available, pack_refs, peel_ref,
create_symref, resolve_gitlink_ref.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: add methods for head_ref*

Add methods for the head_ref* functions.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs.c: add methods for the ref iterators

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
refs-be-files.c: rename refs.c to refs-be-files.c

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>