cached dir content changed message should print out *what* changed

[tych0]

https://github.com/anuvu/stacker/blob/master/cache.go#L260

we have the diff here, we could iterate through it and print all the paths that changed. but there are potentially lots of these, so we should do some reasonable truncating.

[raharper]

Yeah, it could get verbose real quick. I would suggest:

1. if --debug is set, print the whole lot
2. if not --debug then we could print which import entry was modified and mention passing --debug for complete details.

[tych0]

I think it might be reasonable to print maybe the first 1-2 with a (N additional differences) or something, just to jog peoples' memory. But yeah, I agree that --debug should print everything.