On filesystems supporting copy-on-write, like Btrfs and ZFS, use
"cp --reflink" to achieve faster speed and lower disk space usage.

Option --reflink and --no-reflink can override the automatic detection.

When reflink is supported, toolchains, support libraries, and artifacts
would also be preserved. This means you can do incremental builds right
after creating a new workdir.

For Linux Chromium checkout, one additional workdir only takes 0.8~1 GB
disk space in Btrfs, including data and metadata, depending on how many
artifacts are there inside the out*/ directories.

Change-Id: I8b913efba9e5afaeea2adc1c10a561f63cb50282
Bug: 721585
Reviewed-on: https://chromium-review.googlesource.com/499567
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>