This new function will replace "read_sha1_file". This latter function
becoming just a stub to call the former will a NULL "replacement"
argument.

This new function is needed because sometimes we need to use the
replacement sha1.
Signed-off-by: NChristian Couder <chriscool@tuxfamily.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>