-Ytest-pickler: avoid forcing anything when there's a difference

This might cause a stale symbol exception and make it harder to find the
source of the problem.

Author: smarter

compiler/src/dotty/tools/dotc/transform/Pickler.scala

@@ -93,7 +93,7 @@ class Pickler extends Phase {
     if (previous != unpickled) {
       output("before-pickling.txt", previous)
       output("after-pickling.txt", unpickled)
-      ctx.error(i"""pickling difference for ${cls.fullName} in ${cls.sourceFile}, for details:
+      ctx.error(s"""pickling difference for ${cls} in ${cls.sourceFile}, for details:
                    |
                    |  diff before-pickling.txt after-pickling.txt""")
     }