If the repo is a btrfs subvolume, take a snapshot instead of using
"cp --reflink". Taking a snapshot is much faster, and uses less
disk space for metadata.

Bug: 721585
Change-Id: I97753dc30d46ff56d84d65f8d815c19d09cf104b
Reviewed-on: https://chromium-review.googlesource.com/513586Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>