R1:347252fda85a

Improve clarity of commit and symbol handling in DiffusionRequest

Summary:
Ref T2683. Currently, DiffusionRequest has four different "commitey" things:

- `commit`
- `rawCommit`
- `symbolicCommit`
- `stableCommit`

Of these, only two are actually distinct, useful values: `symbolicCommit` (which holds the value the request originally contained, if one existed) and `stableCommit` (which resolves that value, or the value implied by its omission, into a stable, permanent commit identifier).