gitweb: Refactor git_header_html

Extract the following parts into separate subroutines:

 * finding correct MIME content type for HTML pages (text/html or
   application/xhtml+xml?) into get_content_type_html()
 * printing <link ...> elements in HTML head into print_header_links()
 * printing navigation "breadcrumbs" for given action into
   print_nav_breadcrumbs()
 * printing search form into print_search_form()

This reduces git_header_html to two pages long (53 lines), making gitweb
code easier to read.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>