strbuf: fix urlencode format string on signed char

Git credential fails with special char in password with

    remote: Invalid username or password.
    fatal: Authentication failed for

    File ~/.git-credential contains badly urlencoded characters
    %ffffffXX%ffffffYY instead of %XX%YY.

Add a cast to an unsigned char to fix urlencode use of %02x on a
char.

Signed-off-by: Julien Dusser <julien.dusser@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.c

@@ -658,7 +658,7 @@ static void strbuf_add_urlencode(struct strbuf *sb, const char *s, size_t len,
 		    (!reserved && is_rfc3986_reserved(ch)))
 			strbuf_addch(sb, ch);
 		else
-			strbuf_addf(sb, "%%%02x", ch);
+			strbuf_addf(sb, "%%%02x", (unsigned char)ch);
 	}
 }