update: refactored notation list command (notaryproject#481)

This PR refactors the `notation list` command reducing the number of
calls to getSignatureRepository() and repo.Resolve().

Signed-off-by: Patrick Zheng <patrickzheng@microsoft.com>

cmd/notation/verify.go

@@ -70,6 +70,13 @@ func runVerify(command *cobra.Command, opts *verifyOpts) error {
 		return err
 	}
 
+	// initialize
+	reference := opts.reference
+	sigRepo, err := getSignatureRepository(ctx, &opts.SecureFlagOpts, reference)
+	if err != nil {
+		return err
+	}
+
 	// resolve the given reference and set the digest
 	ref, err := resolveReference(command.Context(), &opts.SecureFlagOpts, reference, sigRepo, func(ref registry.Reference, manifestDesc ocispec.Descriptor) {
 		fmt.Printf("Resolved artifact tag `%s` to digest `%s` before verification.\n", ref.Reference, manifestDesc.Digest.String())