gitweb: Add startup delay to activity indicator for cache

gitweb: Add startup delay to activity indicator for cache

This adds support for [optional] startup delay to git_generating_data_html()
subroutine, which is used to provide "Generating..." page as activity
indicator when waiting for page to be generated if caching.  If the data
(page contents) gets generated within $generating_options{'staryp_delay'}
seconds, the "Generating..." page won't get displayed.

This feature was created in response to complaint by Petr 'Pasky' Baudis'
about "Generating..." feature.

NOTE: This startup delay allows to use git_generating_data_html() also
for process generating data, assuming that die_error(), which turns of
capturing and which output (error page) is not cache, finishes within
startup delay.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>