git-instaweb: Simplify build dependency on gitweb

Since c0cb4ed (git-instaweb: Configure it to work with new gitweb structure, 2010-05-28) git-instaweb does not re-create gitweb.cgi etc., but makes use of installed gitweb. Therefore simplify git-instaweb dependency on gitweb subsystem in main Makefile from 'gitweb/gitweb.cgi gitweb/static/gitweb.css gitweb/static/gitweb.js' to simply 'gitweb'. This is preparation for splitting gitweb.perl script, and for splitting gitweb.js (to be reassembled / combined on build). This way we don't have to duplicate parts of gitweb/Makefile in main Makefile... it is also more correct description of git-instaweb dependency. Reported-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Jakub Narebski <jnareb@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>