http.c: remove verification of remote packs

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>