Add a lockfile function to append to a file

Add a lockfile function to append to a file

This takes care of copying the original contents into the replacement
file after the lock is held, so that concurrent additions can't miss
each other's changes.

[jc: munged to drop mmap in favor of copy_file.]

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>