gitweb: Git config keys are case insensitive, make config search too

gitweb: Git config keys are case insensitive, make config search too

"git config -z -l" that gitweb uses in git_parse_project_config() to
populate %config hash returns section and key names of config
variables in lowercase (they are case insensitive).  When checking
%config in git_get_project_config() we have to take it into account.

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