Fix copy without any flag set

PR sigstore#3247 added an --only flag which when set copies only a subset of
metadata. By default the flag is set to an empty string, meaning that by
default copy would copy nothing. This PR fixes this bug so that if only
is unset, copy defaults to the same behavior as before copying
everything.

Also update the tag name to be sig rather than sign for the flag.

Signed-off-by: Hayden Blauzvern <hblauzvern@google.com>

cmd/cosign/cli/copy.go

@@ -34,10 +34,10 @@ func Copy() *cobra.Command {
   cosign copy example.com/src:latest example.com/dest:latest
 
   # copy the signatures only
-  cosign copy --only=sign example.com/src example.com/dest
+  cosign copy --only=sig example.com/src example.com/dest
 
   # copy the signatures, attestations, sbom only
-  cosign copy --only=sign,att,sbom example.com/src example.com/dest
+  cosign copy --only=sig,att,sbom example.com/src example.com/dest
 
   # overwrite destination image and signatures
   cosign copy -f example.com/src example.com/dest

cmd/cosign/cli/copy/copy.go

@@ -79,7 +79,13 @@ func CopyCmd(ctx context.Context, regOpts options.RegistryOptions, srcImg, dstIm
 		return err
 	}
 
+	onlyFlagSet := false
 	tags := parseOnlyOpt(copyOnly, sigOnly)
+	if len(tags) > 0 {
+		onlyFlagSet = true
+	} else {
+		tags = []tagMap{ociremote.SignatureTag, ociremote.AttestationTag, ociremote.SBOMTag}
+	}
 	if err := walk.SignedEntity(gctx, root, func(ctx context.Context, se oci.SignedEntity) error {
 		// Both of the SignedEntity types implement Digest()
 		h, err := se.Digest()
@@ -126,7 +132,7 @@ func CopyCmd(ctx context.Context, regOpts options.RegistryOptions, srcImg, dstIm
 	}
 
 	// If we're only copying sig/att/sbom, we have nothing left to do.
-	if len(tags) > 0 {
+	if onlyFlagSet {
 		return nil
 	}
 
@@ -174,17 +180,16 @@ func remoteCopy(ctx context.Context, pusher *remote.Pusher, src, dest name.Refer
 	return pusher.Push(ctx, dest, got)
 }
 
-func parseOnlyOpt(str string, sigOnly bool) []tagMap {
+func parseOnlyOpt(onlyFlag string, sigOnly bool) []tagMap {
 	var tags []tagMap
-	items := strings.Split(str, ",")
-	tagSet := sets.New(items...)
+	tagSet := sets.New(strings.Split(onlyFlag, ",")...)
 
 	if sigOnly {
-		fmt.Fprintf(os.Stderr, "--sig-only is deprecated, use --only=sign instead")
+		fmt.Fprintf(os.Stderr, "--sig-only is deprecated, use --only=sig instead")
 		tagSet.Insert("sign")
 	}
 
-	if tagSet.Has("sign") {
+	if tagSet.Has("sig") {
 		tags = append(tags, ociremote.SignatureTag)
 	}
 	if tagSet.Has("sbom") {