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

diff --cc Makefile
Simple merge

diff --cc cache.h
Simple merge

diff --cc path.c
Simple merge

diff --cc refs.c
Simple merge