Fix an issue with `differential.getdiff` when providing a revision ID

Summary:
If handed a revision ID, we might get more than one result, which causes `executeOne()` to throw. Instead, translate the revision id into a diff ID before querying for the diff.

Also one small consistency change to parameter casing.

Test Plan: Used console to query for a revision with more than one diff using the revision id.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, mbishopim3

Differential Revision: https://secure.phabricator.com/D7026