Throw InvalidObjectIdException from ObjectId.fromString("tooshort")

Throw InvalidObjectIdException from ObjectId.fromString("tooshort")

ObjectId.fromString already throws InvalidObjectIdException for most
malformed object ids, but for this kind it previously threw
IllegalArgumentException.  Since InvalidObjectIdException is a child of
IllegalArgumentException, callers that catch IllegalArgumentException
will continue to work.

Change-Id: I24e1422d51607c86a1cb816a495703279e461f01
Signed-off-by: Jonathan Nieder <jrn@google.com>