http.c: remove verification of remote packs

Make http.c::fetch_pack_index() no longer check for the remote pack
with a HEAD request before fetching the corresponding pack index file.

Not only does sending a HEAD request before we do a GET incur a
performance penalty, it does not offer any significant error-
prevention advantages (pack fetching in the *_http_pack_request()
methods is capable of handling any errors on its own).

This addresses an issue raised elsewhere:

  http://code.google.com/p/msysgit/issues/detail?id=323
  http://support.github.com/discussions/repos/957-cant-clone-over-http-or-git

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

@@ -869,17 +869,6 @@ static int fetch_pack_index(unsigned char *sha1, const char *base_url)
 	char *url;
 	struct strbuf buf = STRBUF_INIT;
 
-	/* Don't use the index if the pack isn't there */
-	end_url_with_slash(&buf, base_url);
-	strbuf_addf(&buf, "objects/pack/pack-%s.pack", hex);
-	url = strbuf_detach(&buf, 0);
-
-	if (http_get_strbuf(url, NULL, 0)) {
-		ret = error("Unable to verify pack %s is available",
-			    hex);
-		goto cleanup;
-	}
-
 	if (has_pack_index(sha1)) {
 		ret = 0;
 		goto cleanup;