c8d/pull: Use same progress action as distribution

Docker with containerd integration emits "Exists" progress action when a
layer of the currently pulled image already exists. This is different
from the non-c8d Docker which emits "Already exists".

This makes both implementations consistent by emitting backwards
compatible "Already exists" action.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

daemon/containerd/progress.go

@@ -152,7 +152,7 @@ func (p pullProgress) UpdateProgress(ctx context.Context, ongoing *jobs, out pro
 		} else if p.ShowExists {
 			out.WriteProgress(progress.Progress{
 				ID:         stringid.TruncateID(j.Digest.Encoded()),
-				Action:     "Exists",
+				Action:     "Already exists",
 				HideCounts: true,
 				LastUpdate: true,
 			})