loader: Clean replay directory before post_load operation
The post_load
operation will export the last loaded revision to a
new temporary directory to check possible revision divergence.
However the reconstructed filesystem for that revision still exists in another temporary directory after all revisions have been replayed.
So ensure to clean that latter before post_load to gain some disk space
and avoid possible "No space left on device"
errors.
Migrated from D6721 (view on Phabricator)