commit, payload-reflink: do not write to the parent repo

commit, payload-reflink: do not write to the parent repo

reintroduce the feature that was reverted with commit:

28c7bc6d0e153a0b07bdb82d25473a490765067f

Differently than the original implementation, now we don't attempt any
test for reflinks support on the parent repository, since the test
requires write access to the repository.

Additionally, also check that the two repositories are on the same
device before attempting any reflink.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
Closes: #1525
Approved by: cgwalters