GVFS prefetch should support --file-list

[turbonaitis]

1. In a scenario, where my build tool (like Buck, or Bazel), can give me a list of input files, needed to build a target, it would be great to be able to provided this list of files via an input file, instead of having to pass all of them in as a command-line argument
2. It would be even better, if --file-list would accept that list via stdin, if no input file is specified, so one could do

getListOfInputs | gvfs prefetch --file-list