Preserve file permissions when unpacking sources

[Fuuzetsu]

The tar package, in name of portability, doesn't preserve file
permissions when creating and unpacking archives. A sample package which
relies on permissions to be kept as they were packed with is
ghc-parser which executes a bash script as part of its build process.

Before this patch we'd get

    /run/user/1000/stack26234/ghc-parser-0.1.7.0/Setup.hs:2:1: Warning:
        Module ‘System.Cmd’ is deprecated: Use "System.Process" instead
    /bin/sh: ./build-parser.sh: Permission denied
    Configuring ghc-parser-0.1.7.0...
    Warning: 'hs-source-dirs: src-7.8.3' directory does not exist.

but now the package builds fine.

cc @mboes

[snoyberg]

Testing on Windows before merging, but this looks good.

[snoyberg]

Hmm, I'm getting errors like this when running the integration test on Windows:

setPermissions: does not exist (No such file or directory)

Do you have access to a Windows system to test this on?

[Fuuzetsu]

@snoyberg No Windows machine access. I'm not sure why that problem would occur: FP.</> uses native separator, and fromTarPath (https://hackage.haskell.org/package/tar-0.4.2.0/docs/Codec-Archive-Tar-Entry.html#v:fromTarPath) is documented to convert to native separator. Simple way to check would be to print the path we get but as I said, I don't have access to a Windows machine. I'd be leaning towards fromTarPath not doing its job properly but can't check.

[snoyberg]

It seems like Windows doesn't like setting permissions on a directory when a trailing backslash is present. I think I can fix this easily.

[snoyberg]

Fix added for Windows and merged, thanks!