server/sync: add sha-224 to the sync'd prefixes

Updates #1172

Change-Id: Ic9e927aad5f90e926bd50a5ab4a17d540c3f5a2b
diff --git a/pkg/server/sync.go b/pkg/server/sync.go
index c13761e..6375497 100644
--- a/pkg/server/sync.go
+++ b/pkg/server/sync.go
@@ -894,9 +894,10 @@
 	var pfx []string
 	// TODO(bradfitz): do limit=1 enumerates against sh.from and sh.to with varying
 	// "after" values to determine all the blobref types on both sides.
-	// For now, be lazy and assume only sha1:
+	// For now, be lazy and assume only sha1 and sha224:
 	for i := 0; i < 256; i++ {
 		pfx = append(pfx, fmt.Sprintf("sha1-%02x", i))
+		pfx = append(pfx, fmt.Sprintf("sha224-%02x", i))
 	}
 	return pfx
 }