gitweb: Make git_search_* subroutines render whole pages

gitweb: Make git_search_* subroutines render whole pages

Move git_header_html() and git_footer_html() invocation from git_search()
to individual git_search_* subroutines.

While at it, reorganize search-related code a bit, moving invoking of git
commands before any output is generated.

This has the following advantages:

  * gitweb now shows an error page if there was unknown search type
    (evaluate_and_validate_params checks only that it looks sanely);
    remember that we shouldn't call die_error after any output.

  * git_search_message is now safe agains die_error in parse_commits
    (though this is very unlikely).

  * gitweb now can check errors while invoking git commands and show
    error page (again, quite unlikely).

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>