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

gitweb/gitweb.perl

@@ -1000,8 +1000,16 @@ sub handle_errors_html {
 	die_error(400, "Project needed");
 }
 $actions{$action}->();
+
 DONE_GITWEB:
-1;
+if (defined caller) {
+	# wrapped in a subroutine processing requests,
+	# e.g. mod_perl with ModPerl::Registry, or PSGI with Plack::App::WrapCGI
+	return;
+} else {
+	# pure CGI script, serving single request
+	exit;
+}
 
 ## ======================================================================
 ## action links