gitweb: Separate generating 'sort by' table header

gitweb: Separate generating 'sort by' table header

Extract generating table header cell, for tables which can be sorted
by its columns, into print_sort_th_str() and print_sort_th_num()
subroutines, and print_sort_th() driver subroutine.

This avoids repetition, and should make further improvements (like
JavaScript sorting) easier.  The subroutine uses now "replay" link,
so it is generic enough to be able to use it for other tables which
can be sorted by column, like for example 'heads' and 'tags' view
(sort by name, or sort by age).

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