Be less verbose when extracting GHC

[ndmitchell]

Given stack --arch i386 --no-terminal --install-ghc build, that produces about 20K lines of output of the form Extracting .... That level of output makes it impossible to find anything in my console, and also means that for CI system (e.g. appveyor) it becomes ridiculously sluggish to navigate. Perhaps pass a quieter option to 7zip? Updates every 10 seconds on how many files have been extracted so far would be fine, or a . for each file, or something else.

[ndmitchell]

For an example of sluggish and huge output https://ci.appveyor.com/project/ndmitchell/neil/build/1.0.1

[ndmitchell]

Current workaround I use is stack setup > nul, which produces an eclectic selection of messages. Just about enough for me to know where it's got to, but not a sensibly chosen set.

[borsboom]

I agree, it's really annoying. I don't see a command-line option for 7z to disable all the output. I think we'll have to swallow the stdout in this code. Might be as simple as switching to using readProcessNull instead of rawSystem. A PR would be welcome.

[ndmitchell]

A better option would be to swallow the lines and turn it into a single . per line, or even better, a single . per 1% of files.

[snoyberg]

Opened #4643