Storage drivers may be able to take advantage of the hint to start
their walk more efficiently.
For S3: The API takes a start-after parameter. Registries with many
repositories can drastically reduce calls to s3 by telling s3 to only
list results lexographically after the last parameter.
For the fallback: We can start deeper in the tree and avoid statting
the files and directories before the hint in a walk. For a filesystem
this improves performance a little, but many of the API based drivers
are currently treated like a filesystem, so this drastically improves
the performance of GCP and Azure blob.
Signed-off-by: James Hewitt <james.hewitt@uk.ibm.com>
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