Bugfix: Fix rare 'not found in repository' error for copy command

In rare cases copy (and other commands) could report that LoadTree(...)
returned a `id [...] not found in repository` error. This could be caused by a
backup or copy command running concurrently. The error is only temporary,
running the failed restic command a second time as a workaround solves the
error.

This issue has been fixed by correcting the order in which restic reads data
from the repository. It is now guaranteed that restic only loads snapshots for
which all necessary data is already available.

https://github.com/restic/restic/issues/3432
https://github.com/restic/restic/pull/3570
