gitweb: Navbar refactoring - use git_page_nav to generate navigation bar

gitweb: Navbar refactoring - use git_page_nav to generate navigation bar

Use git_page_nav subroutine to generate navigation bar.  Additional
navigation (either formats or pager/pagination) is put into variables.

Corrects error in git_search where hash parameter was added to
"summary" link instead of to "log" link.  Might differ from previous
version by additional "<br/>" in navigation bar.

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