git-instaweb: Simplify build dependency on gitweb

Since c0cb4ed3 (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: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -1773,7 +1773,7 @@ gitweb/static/gitweb.min.css: gitweb/static/gitweb.css
 endif # CSSMIN
 
 
-git-instaweb: git-instaweb.sh gitweb/gitweb.cgi gitweb/static/gitweb.css gitweb/static/gitweb.js
+git-instaweb: git-instaweb.sh gitweb
 	$(QUIET_GEN)$(RM) $@ $@+ && \
 	sed -e '1s|#!.*/sh|#!$(SHELL_PATH_SQ)|' \
 	    -e 's/@@GIT_VERSION@@/$(GIT_VERSION)/g' \