repo: Revert default timestamp from 1 back to 0

repo: Revert default timestamp from 1 back to 0

Quoting Dan Nicholson in

  <https://github.com/ostreedev/ostree/pull/330#issuecomment-245499099>

  mtime of 0 has been the semantics of ostree deployments from basically
  the beginning of the project. We (and others, see
  flatpak/flatpak@b5204c9) rely on that fact when generating trees.

  In particular, this affects caches that use the mtime of the
  associated file or directory to determine if the cache is valid. By
  arbitrarily changing the mtime of the files to something else, all
  the caches we setup in the build are now invalidated. Preseeding
  caches is really important to the user experience as it avoids
  having the user wait while they're regenerated on first run.

  Now, we could change our build infrastructure to preset all the
  mtimes to 1 to match this change, but what does that do for our
  existing users who are on an ostree that deploys with mtimes of 0?
  We could just revert this change at Endless (and the associated one
  in Flatpak), and that would be fine for our users. However, if we
  point non-Endless users to our apps, they'll have the great
  experience of waiting 10 seconds the first time they launch it while
  the fontconfig cache is rebuilt unnecessarily.

Closes: #495
Approved by: jlebon