Migrate to optional_field_ref Thrift API

Summary:
Migrate the code from accessing optional Thrift fields directly to a safer
`optional_field_ref` API. See https://fburl.com/safe for more details.

The output of this codemod has been reviewed in D13259011.

To preserve semantics, each unchecked access is replaced with an explicit call
to `value_unchecked()`. If you are sure that accessing a field is safe (the
field is marked as set), you can later replace `value_unchecked()` with
`value()` or dereferencing (`operator *`):

```
  ThriftStruct s = ...
- auto foo = s.foo_ref().value_unchecked();
+ auto foo = *s.foo_ref(); // will throw if s.foo is unset
```

drop-conflicts

Reviewed By: stevegury

Differential Revision: D15786632

fbshipit-source-id: 7c3f089dfa79597b189da6996cb76dcdbd5ed612

eden/fs/store/mononoke/MononokeThriftBackingStore.cpp

@@ -97,7 +97,7 @@ folly::Future<std::unique_ptr<Tree>> MononokeThriftBackingStore::getTree(
                 Hash(file.hash.hash),
                 file.name,
                 treeEntryTypeFromMononoke(file.file_type),
-                file.size,
+                file.size_ref().value_unchecked(),
                 Hash(file.content_sha1_ref().value_unchecked()));
           } else {
             entries.emplace_back(