Attempt layer mounts from up to 3 source repositories, possibly
falling back to a standard blob upload for cross repository pushes.
Addresses compatiblity issues with token servers which do not grant
multiple repository scopes, resulting in an authentication failure for
layer mounts, which would otherwise cause the push to terminate with an
error.
Signed-off-by: Brian Bland <brian.bland@docker.com>
1d3480f9ba