Remove uses of deprecated go-digest.NewDigestFromHex, go-digest.Digest.Hex

Both of these were deprecated in 55f675811a,
but the format of the GoDoc comments didn't follow the correct format, which
caused them not being picked up by tools as "deprecated".

This patch updates uses in the codebase to use the alternatives.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

content/local/store_test.go

@@ -308,7 +308,7 @@ func checkBlobPath(t *testing.T, cs content.Store, dgst digest.Digest) string {
 		t.Fatalf("failed to calculate blob path: %v", err)
 	}
 
-	if path != filepath.Join(cs.(*store).root, "blobs", dgst.Algorithm().String(), dgst.Hex()) {
+	if path != filepath.Join(cs.(*store).root, "blobs", dgst.Algorithm().String(), dgst.Encoded()) {
 		t.Fatalf("unexpected path: %q", path)
 	}
 	fi, err := os.Stat(path)