Merge branch 'dt/refs-backend-pre-vtable'

Code preparation for pluggable ref backends. * dt/refs-backend-pre-vtable: refs: break out ref conflict checks files_log_ref_write: new function initdb: make safe_create_dir public refs: split filesystem-based refs code into a new file refs/refs-internal.h: new header file refname_is_safe(): improve docstring pack_if_possible_fn(): use ref_type() instead of is_per_worktree_ref() copy_msg(): rename to copy_reflog_msg() verify_refname_available(): new function verify_refname_available(): rename function