* `CodeLocation` had a `blob` and `commit` field. These are redundant,
since the review lives under a revision.
We also change the line range to be diff-compatible, with an old and
new range. The ranges are optional to account for different scenarios
like file deletion, move etc.
* `CodeComment` had redundant getters which were removed in favor of
public fields. We also remove the `timestamp` field, as it will be
the same as the review timestamp.
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