This backports the listing-side changes of HADOOP-13230.
With this patch in, this branch of Hadoop is compatible with S3A clients
which do not delete directory markers when files are created underneath.
It does not allow this version to disable marker deletion; if the
fs.s3a.marker.retention option is changed to request this, a message
is printed at INFO and the policy remains at "delete"
The s3guard bucket-info command has been extended to support
probing for marker retention, as has the hasPathCapability method on
S3AFileSystem.
Read the documentation!
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