tar: set mtime and gid independently
 [rsc] --rw-rw-r-- M 467740 glenda sys 24068 Mar 12 09:15 sys/src/cmd/tar.c
	/n/sourcesdump/2006/0312/plan9/sys/src/cmd/tar.c:1027,1035 - 
	/n/sourcesdump/2006/0313/plan9/sys/src/cmd/tar.c:1027,1038
	  
	  			nulldir(&nd);
	  			nd.mtime = mtime;
	- 			if (isustar(hp))
	- 				nd.gid = hp->gname;
	  			dirfwstat(fd, &nd);
	+ 			if (isustar(hp)) {
	+ 				nulldir(&nd);
	+ 				nd.gid = hp->gname;
	+ 				dirfwstat(fd, &nd);
	+ 			}
	  		}
	  		close(fd);
	  	}