piping from/to subprocesses is extremely slow on Windows

[floitsch]

The following function takes ~100 seconds to copy the SDK (~385 files and 94MB). On Linux the whole test is finished in 3 seconds.

copy_directory --source/string --target/string:
  directory.mkdir --recursive target
  source = source.replace --all "\\" "/"
  target = target.replace --all "\\" "/"
  // We are using `tar` so we keep the permissions.
  tar := tool_path_ "tar"
  from := pipe.from [tar, "c", "-f", "-", "-C", source, "."]
  to := pipe.to [tar, "x", "-f", "-", "-C", target]
  writer := writer.Writer to
  while chunk := from.read:
    writer.write chunk
  from.close
  to.close

tool_path_ function

tool_path_ tool/string -> string:
  if platform != PLATFORM_WINDOWS: return tool
  // On Windows, we use the <tool>.exe that comes with Git for Windows.

  // TODO(florian): depending on environment variables is brittle.
  // We should use `SearchPath` (to find `git.exe` in the PATH), or
  // 'SHGetSpecialFolderPath' (to find the default 'Program Files' folder).
  program_files_path := os.env.get "ProgramFiles"
  if not program_files_path:
    // This is brittle, as Windows localizes the name of the folder.
    program_files_path = "C:/Program Files"
  result := "$program_files_path/Git/usr/bin/$(tool).exe"
  if not file.is_file result:
    throw "Could not find $result. Please install Git for Windows"
  return result