diff command generates panic

[offthewoll]

I've just updated to the latest version of drive minutes ago and got a panic when trying to use the diff command. I'm using the Go packaged with Ubuntu 14.04.

$ drive diff list2html.py 
panic: not implemented

goroutine 1 [running]:
runtime.panic(0x638860, 0xc21004b800)
        /usr/lib/go/src/pkg/runtime/panic.c:266 +0xb6
github.com/rakyll/drive.(*Commands).Diff(0xc21004f760, 0xc21004f740, 0xc21004f760)
        /home/ubuntu/go/src/github.com/rakyll/drive/diff.go:18 +0x55
main.(*diffCmd).Run(0xa449d8, 0xc21000a020, 0x1, 0x1)
        /home/ubuntu/go/src/github.com/rakyll/drive/cmd/drive/main.go:111 +0xaf
github.com/rakyll/command.Run()
        /home/ubuntu/go/src/github.com/rakyll/command/command.go:155 +0x81
github.com/rakyll/command.ParseAndRun()
        /home/ubuntu/go/src/github.com/rakyll/command/command.go:162 +0x1f
main.main()
        /home/ubuntu/go/src/github.com/rakyll/drive/cmd/drive/main.go:45 +0x83c

[rakyll]

It has never worked before. The panic message explains it well, it should be implemented.

[offthewoll]

Got it, thank you. Should the command maybe print a message to the user stating so, until the feature is implemented?

[odeke-em]

I started a PR to address this, please take a look at #39