gitweb: Refactor reading and parsing config file into read_config_file

gitweb: Refactor reading and parsing config file into read_config_file

Beside being obvious reduction of duplicated code, this is enables us
to easily call site-wide config file in per-installation config file.

The actual update to documentation is left for next commit, because of
possible exclusive alternative (possible other next commit) of always
reading system-wide config file and relying on per-instalation config
file overriding system-wide defaults.

Signed-off-by: Jakub Narebski <redacted>
Acked-by: John 'Warthog9' Hawley <redacted>
Signed-off-by: Junio C Hamano <redacted>