Add hash_sha1_file()

Most callers of write_sha1_file_prepare() are only interested in the
resulting hash but don't care about the returned file name or the header.
This patch adds a simple wrapper named hash_sha1_file() which does just
that, and converts potential callers.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>