Fix --signoff in builtin-commit differently.

Introduce fmt_name() specifically meant for formatting the name and
email pair, to add signed-off-by value.  This reverts parts of
13208572 (builtin-commit: fix --signoff)
so that an empty datestamp string given to fmt_ident() by mistake will
error out as before.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -346,11 +346,9 @@ static int prepare_log_message(const char *index_file, const char *prefix)
 
 		strbuf_init(&sob, 0);
 		strbuf_addstr(&sob, sign_off_header);
-		strbuf_addstr(&sob, fmt_ident(getenv("GIT_COMMITTER_NAME"),
-					      getenv("GIT_COMMITTER_EMAIL"),
-					      "", 1));
+		strbuf_addstr(&sob, fmt_name(getenv("GIT_COMMITTER_NAME"),
+					     getenv("GIT_COMMITTER_EMAIL")));
 		strbuf_addch(&sob, '\n');
-
 		for (i = sb.len - 1; i > 0 && sb.buf[i - 1] != '\n'; i--)
 			; /* do nothing */
 		if (prefixcmp(sb.buf + i, sob.buf)) {

cache.h

@@ -444,6 +444,7 @@ enum date_mode parse_date_format(const char *format);
 extern const char *git_author_info(int);
 extern const char *git_committer_info(int);
 extern const char *fmt_ident(const char *name, const char *email, const char *date_str, int);
+extern const char *fmt_name(const char *name, const char *email);
 
 struct checkout {
 	const char *base_dir;

ident.c

@@ -192,12 +192,14 @@ static const char *env_hint =
 "Omit --global to set the identity only in this repository.\n"
 "\n";
 
-const char *fmt_ident(const char *name, const char *email,
-		      const char *date_str, int error_on_no_name)
+static const char *fmt_ident_1(const char *name, const char *email,
+			       const char *date_str, int flag)
 {
 	static char buffer[1000];
 	char date[50];
 	int i;
+	int error_on_no_name = !!(flag & 01);
+	int name_addr_only = !!(flag & 02);
 
 	setup_ident();
 	if (!name)
@@ -224,24 +226,36 @@ const char *fmt_ident(const char *name, const char *email,
 	}
 
 	strcpy(date, git_default_date);
-	if (date_str) {
-		if (*date_str)
-			parse_date(date_str, date, sizeof(date));
-		else
-			date[0] = '\0';
-	}
+	if (!name_addr_only && date_str)
+		parse_date(date_str, date, sizeof(date));
 
 	i = copy(buffer, sizeof(buffer), 0, name);
 	i = add_raw(buffer, sizeof(buffer), i, " <");
 	i = copy(buffer, sizeof(buffer), i, email);
-	i = add_raw(buffer, sizeof(buffer), i, date[0] ? "> " : ">");
-	i = copy(buffer, sizeof(buffer), i, date);
+	if (!name_addr_only) {
+		i = add_raw(buffer, sizeof(buffer), i,  "> ");
+		i = copy(buffer, sizeof(buffer), i, date);
+	} else {
+		i = add_raw(buffer, sizeof(buffer), i, ">");
+	}
 	if (i >= sizeof(buffer))
 		die("Impossibly long personal identifier");
 	buffer[i] = 0;
 	return buffer;
 }
 
+const char *fmt_ident(const char *name, const char *email,
+		      const char *date_str, int error_on_no_name)
+{
+	int flag = (error_on_no_name ? 01 : 0);
+	return fmt_ident_1(name, email, date_str, flag);
+}
+
+const char *fmt_name(const char *name, const char *email)
+{
+	return fmt_ident_1(name, email, NULL, 03);
+}
+
 const char *git_author_info(int error_on_no_name)
 {
 	return fmt_ident(getenv("GIT_AUTHOR_NAME"),