Align "RevisionQuery->needCommitPHIDs()" to use Edges, not the legacy table

Summary:
Depends on D20458. Ref T13276. Although I'm not thrilled about "needCommitPHIDs()", it has a few callers, including custom fields. Allow "need + attach + get" to survive for now since they're reasonably modern, at least.

However, use edges instead of "TABLE_COMMIT" and require need...() + get...(), removing the direct load...().

Also remove RevisionQuery->withCommitPHIDs(...), which has no callers.

Test Plan:

• Grepped for loadCommitPHIDs (only two hits, the private RevisionQuery method).
• Called "differential.getrevision", got commits.
• Viewed a revision, saw "Commits: ...".
• Grepped for withCommitPHIDs(), no callers on RevisionQuery (some other query classes have methods with this name).
• Called "differential.query", got commits.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13276

Differential Revision: https://secure.phabricator.com/D20459