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).

• rawCommit is equivalent to symbolicCommit and can be simply removed.
• commit has some sketchy magic around it that needs to be pulled out before it can be jettisoned.

Test Plan: Viewed SVN, Git, and Mercurial repositories. Viewed brwose/history/change/tag/branch/etc views.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9098