sha1_to_hex: properly terminate the SHA1

sha1_to_hex: properly terminate the SHA1

sha1_to_hex() returns a pointer to a static buffer. Some of its users
modify that buffer by appending a newline character. Other users rely
on the fact that you can call

	printf("%s", sha1_to_hex(sha1));

Just to be on the safe side, terminate the SHA1 in sha1_to_hex().

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>