Improve bytecode version diff

[Goooler]

OLD: old.jar
NEW: new.jar

 JAR   │ old      │ new      │ diff   
───────┼──────────┼──────────┼────────
 class │ 1.19 KiB │ 1.35 KiB │ +166 B 
 other │     72 B │     72 B │    0 B 
───────┼──────────┼──────────┼────────
 total │ 1.26 KiB │ 1.42 KiB │ +166 B 

 CLASSES │ old │ new │ diff       
─────────┼─────┼─────┼────────────
 classes │   1 │   1 │  0 (+0 -0) 
 methods │   4 │   5 │ +1 (+1 -0) 
  fields │   1 │   1 │  0 (+0 -0) 

=================
====   JAR   ====
=================

 size     │ diff   │ path                       
──────────┼────────┼────────────────────────────
 1.35 KiB │ +166 B │ ∆ org/example/MainKt.class 
──────────┼────────┼────────────────────────────
 1.35 KiB │ +166 B │ (total)                    


=====================
====   CLASSES   ====
=====================

BYTECODE VERSIONS:

   version │ old │ new │ diff
  ─────────┼─────┼─────┼────────────
        65 │   1 │   0 │ -1 (+0 -1)
        69 │   0 │   1 │ +1 (+1 -0)

  org.example.MainKt: 65 → 69


METHODS:

   old │ new │ diff       
  ─────┼─────┼────────────
   4   │ 5   │ +1 (+1 -0) 
  
  + org.example.MainKt main2()

Closes #490.

[JakeWharton]

I'm not too sure about this behavior. I'm tempted to say that if we show the diff at all, we want to show all counts just for the clarity of it.

[Goooler]

Removed.

[JakeWharton]

Shouldn't this test fail with the filter removed? I would have expected the chart here now...

[Goooler]

versionCounts is emptyMap in this case.