* 'jh/gitweb-caching' (early part):
gitweb: Add optional extra parameter to die_error, for extended explanation
gitweb: add a "string" variant of print_sort_th
gitweb: add a "string" variant of print_local_time
gitweb: Check that $site_header etc. are defined before using them
gitweb: Makefile improvements
gitweb: Load checking
gitweb: Make running t9501 test with '--debug' reliable and usable