We now compute the commit type based on the type of the value and
the type of the parents.
For the first commit we get:
```
struct Commit {
parents: Set<Ref<Cycle<0>>>,
value: T,
}
```
As long as we continue to commit values with type T that type stays
the same.
When we later commits a value of type U we get:
```
struct Commit {
parents: Set<Ref<struct Commit {
parents: Set<Ref<Cycle<0>>>,
value: T | U
}>>,
value: U,
}
```
The new type gets combined as a union type for the value of the inner
commit struct.
Fixes#1495
