gitweb: Add optional "time to generate page" info in footer

gitweb: Add optional "time to generate page" info in footer

Add "This page took XXX seconds and Y git commands to generate"
to page footer, if global feature 'timed' is enabled (disabled
by default).  Requires Time::HiRes installed for high precision
'wallclock' time.

Note that Time::HiRes is being required unconditionally; this is
because setting $t0 variable needs to be done fairly early to have
running time of the whole script.  If Time::HiRes module were required
only if 'timed' feature is enabled, the earliest place where starting
time ($t0) could be calculated would be after reading gitweb config,
making "time to generate page" info inaccurate.

This code is based on example code by Petr 'Pasky' Baudis.

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