Fix search form generation to not modify $cgi->param(...)'s.

In git_header_html() we used to use $cgi->hidden(-name => "a") etc. to
generate hidden fields; unfortunately to use this form it is required
to modify $cgi->param("a") etc., which makes href(-replay,...) use
wrong replay values.  This for example made the "next" link on the
bottom of the page has a=search instead of a=$action, and thus fails to
get you to the next page.

Because in CGI the value of a hidden field is "sticky", there is no
way to modify it short of modifying $cgi->param(...).  Therefore it
got replaced by generating <input type="hidden" ...> element [semi]
directly.

Alternate solution would be for href(-replay,...) to use values saved
in global variables, such as $action etc., instead of (re)reading them
from $cgi->param($symbol).

The bad link was reported by Kai Blin through
  http://bugs.debian.org/481902

Reported-by: Kai Blin <kai.blin@gmail.com>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Tested-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>