make the recording of the files timestamps optional

[thkoch2001]

I'd like to be able to run debootstrap two times (maybe on different machines
or with different configurations) and only see the relevant differences. I don't
really see a use case for the timestamps and they'd add noise to the diff of
two images.

issue first discussed on docker-user: https://groups.google.com/d/msg/docker-user/KdFccS4lw_A/iyj2fPECljgJ

[warpfork]

Definitely agree that being able to take external sources over time and generate meaningful diffs constitutes a win.

Finally got some time to work on it tonight: Epoch timestamps will no longer be serialized by guitar (the code that handles metadata storing), and hroot on master now has an --epoch argument to the build subcommand which will forcibly ignore modtime on the incoming tarstream and instead use epoch.

There's still issues to be resolved with this... namely that it works quite well if you nuke the image in your docker daemon to force a reload from the hroot git repo, but if you don't, the image in docker still has the timestamps, so your next build and commit to the hroot repo without --epoch will be a very noisy diff. Not sure what to do about that yet. Probably remove the image name from docker after a build with the --epoch flag enabled; I can't think of any other more delicate way to modify that metadata.