files_log_ref_write: new function

files_log_ref_write: new function

Because HEAD and stash are per-worktree, every refs backend needs to
go through the files backend to write these refs.

So create a new function, files_log_ref_write, and add it to
refs/refs-internal.h. Later, we will use this to handle reflog updates
for per-worktree symbolic refs (HEAD).

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>