gitweb: use start_form, not startform that was removed in CGI.pm 4.04

gitweb: use start_form, not startform that was removed in CGI.pm 4.04

CGI.pm 4.04 removed the startform method, which had previously been
deprecated in favour of start_form.  Changes file for CGI.pm says:

    4.04 2014-09-04
     [ REMOVED / DEPRECATIONS ]
	- startform and endform methods removed (previously deprecated,
	  you should be using the start_form and end_form methods)

Signed-off-by: Roland Mas <redacted>
Reviewed-by: Jakub Narębski <redacted>
Signed-off-by: Junio C Hamano <redacted>