builtin-commit.c: fix logic to omit empty line before existing footers

"commit -s" used to add an empty line before adding S-o-b line only when
the last line of the existing log message is not another S-o-b line, but
c1e01b0 (commit: More generous accepting of RFC-2822 footer lines.,
2009-10-28) introduced logic to omit this empty line when the message ends
with a run of "footer" lines, to cover S-o-b's friends, e.g. Acked-by.

However, the logic was overzealous and missed one corner case.  A message
that consists of a single line that begins with Token + colon, it can be
mistaken as a S-o-b's friend.  We do want an empty line in such a case.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -530,7 +530,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		for (i = sb.len - 1; i > 0 && sb.buf[i - 1] != '\n'; i--)
 			; /* do nothing */
 		if (prefixcmp(sb.buf + i, sob.buf)) {
-			if (!ends_rfc2822_footer(&sb))
+			if (!i || !ends_rfc2822_footer(&sb))
 				strbuf_addch(&sb, '\n');
 			strbuf_addbuf(&sb, &sob);
 		}