Fix a class variable initialization bug in treediff.

This fixes broken `git checkout`

@devoncarew TBR
Instead of below, you could simply do:
```dart
DiffEntry(this.oldEntry, this.newEntry, this.type,  [this.path]) {
  if (path == null) {
    path = oldEntry == null ? newEntry.name : oldEntry.name;
  }
}
```