gitweb: optionally read config from GITWEB_CONFIG

gitweb: optionally read config from GITWEB_CONFIG

Configuration will first be taken from variables inside the gitweb.cgi
script, which in turn come from the Makefile. Afterwards, the contents of
GITWEB_CONFIG are read, overriding the builtin defaults.

This should eliminate the need for editing the gitweb script at all. Users
should edit the Makefile and/or add a config file.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>