FileStore.backup_file() docstring claimed not to store a
copy of the same file but the behavior of the method did not
match this description.
This commit makes the backed-up file filename derivation
deterministic by hashing its content by SHA-256, thus it
should not back up two files with the same filename and content.
Reviewed-By: Aleksei Slaikovskii <aslaikov@redhat.com>