Sometimes s3 listobjects for a directory includes an entry for that
directory. The restic s3 backend doesn't expect that and returns
an error.
Symptom is:
ReadDir: invalid key name restic/key/, removing prefix
restic/key/ yielded empty string
I'm not sure when s3 does that; I'm unable to reproduce it myself.
But in any case, it seems correct to ignore that when it happens.
Fixes#1068