gitweb: Move generating page title to separate subroutine

gitweb: Move generating page title to separate subroutine

get_page_title subroutine is currently used only in git_header_html.
Nevertheless refactoring title generation allowed to reduce indent
level.

It would be used in more than one callsite in the patch adding caching
activity indicator to gitweb.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Acked-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>