discovery-helper: use reflink copy if available A reflink copy will copy the files "as usual" but keep using the same data block underneath. This is only supported by "copy on write" file system like btrfs or zfs. This will achieve similar performance that the existing hardlink clone that Mercurial performs with the same initial space saving. However, it will behave better on revlogs start being touch by strip. Instead of duplicating all data in the touched revlogs, only the block actually affected by the strip will be duplicated. This save a lot of space when building many variants of large repositories. The --reflink=always flag make sure the `cp` call fails if reflink copies are not supported. Falling back to local clone.

discovery-helper: bail out if destination already exists

discovery-helper: move repository creation in a function This makes it easier to update this duplicated code. (we do a small output fix as we go)

discovery-helper: add an extra argument to generate only one repo This is useful to generate left and right in parallel when dealing with very large repositories.