Don't allocate too much memory in quote_ref_url

In c13b263, http_fetch_ref got "refs/" included in the ref passed to it,
which, incidentally, makes the allocation in quote_ref_url too big, now.

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

http.c

@@ -583,7 +583,7 @@ static char *quote_ref_url(const char *base, const char *ref)
 	int len, baselen, ch;
 
 	baselen = strlen(base);
-	len = baselen + 7; /* "/refs/" + NUL */
+	len = baselen + 2; /* '/' after base and terminating NUL */
 	for (cp = ref; (ch = *cp) != 0; cp++, len++)
 		if (needs_quote(ch))
 			len += 2; /* extra two hex plus replacement % */