fix(diff): heads-only mode should not attempt to parse old refs (#727)

Signed-off-by: Joe Lanford <[email protected]>

Upstream-repository: operator-registry
Upstream-commit: 70d0f510ba911d44917f80e364dcc92e95363e78

Author: joelanford

staging/operator-registry/cmd/opm/alpha/diff/cmd.go

@@ -153,6 +153,8 @@ func (a *diff) parseArgs(args []string) {
 	default:
 		panic("should never be here, CLI must enforce arg size")
 	}
-	a.oldRefs = strings.Split(old, ",")
+	if old != "" {
+		a.oldRefs = strings.Split(old, ",")
+	}
 	a.newRefs = strings.Split(new, ",")
 }