gitweb: Return or exit after done serving request

Check if there is a caller in top frame of gitweb, and either 'return'
if gitweb code is wrapped in subroutine, or 'exit' if it is not.

This should avoid

  gitweb.cgi: Subroutine git_SOMETHING redefined at gitweb.cgi line NNN

warnings in error_log when running gitweb with mod_perl (using
ModPerl::Registry handler)

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>