gitweb/lib - Configure running 'generating_info' when generating data
Add a new 'generating_info_is_safe' cache option; if true, then
process generating data (one that acquired exclusive writer's lock)
would run 'generating_info' if there is no stale data and background
cache generation is enabled.
If function generating (or printing) exits, which leads to cache entry
not being generated (for gitweb this means that there are pages which
are not cached, i.e. error pages), and 'generating_info' also exits,
this could result in bad behavior. Therefore this new option is false
by default.
Updates t9503 test appropriately.
Signed-off-by: Jakub Narebski <jnareb@gmail.com>