update-ref --stdin -z: deprecate interpreting the empty string as zeros

In the original version of this command, for the single case of the
"update" command's <newvalue>, the empty string was interpreted as
being equivalent to 40 "0"s.  This shorthand is unnecessary (binary
input will usually be generated programmatically anyway), and it
complicates the parser and the documentation.

So gently deprecate this usage: remove its description from the
documentation and emit a warning if it is found.  But for reasons of
backwards compatibility, continue to accept it.

Helped-by: Brad King <brad.king@kitware.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/update-ref.c

@@ -154,6 +154,8 @@ static int parse_next_sha1(struct strbuf *input, const char **next,
 				goto invalid;
 		} else if (flags & PARSE_SHA1_ALLOW_EMPTY) {
 			/* With -z, treat an empty value as all zeros: */
+			warning("%s %s: missing <newvalue>, treating as zero",
+				command, refname);
 			hashclr(sha1);
 		} else {
 			/*