format-patch: make newline after signature conditional

When we print an email signature, we print the divider
"-- \n", then the signature string, then two newlines.

Usually the signature is a one-liner (and the default is just the
git version), so the extra newline makes sense.  But one could
easily specify a multi-line signature, like this:

  git format-patch --signature='this is my long signature

  it has multiple lines
  ' ...

and it may end with its own newline, in which case we do not have
to add yet another one.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -844,8 +844,13 @@ static void gen_message_id(struct rev_info *info, char *base)
 
 static void print_signature(void)
 {
-	if (signature && *signature)
-		printf("-- \n%s\n\n", signature);
+	if (!signature || !*signature)
+		return;
+
+	printf("-- \n%s", signature);
+	if (signature[strlen(signature)-1] != '\n')
+		putchar('\n');
+	putchar('\n');
 }
 
 static void add_branch_description(struct strbuf *buf, const char *branch_name)