RefUtil#parseBaseRevision: Do not log an error if baseRevision is invalid
RefUtil#parseBaseRevision: Do not log an error if baseRevision is invalid

Callers should decide whether they want to log the
InvalidRevisionException that is thrown in this case. All current
callers pass in user provided input, if that is invalid it's a 400 Bad
Request, but nothing that should be logged as server error.

Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>
Change-Id: Ifdbac1bccea17f1e45a2c84b8869f9abafbd6518

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