rbt post would always overwrite the value of --guess-summary and

--guess-description with the value of --guess-fields, whether or

not it was actually provided; when -g is not provided, all three

flags will default to auto.
Now we check for the presence of these fields and only overwrite

--guess-summary and --guess-description when they are not provided

and --guess-fields is. Otherwise, when one of --guess-summary or

--guess-description is not provided, it will be set to the value of

--guess-fields.

Ran rbt post with different values of --guess-fields,

--guess-summary, and --guess-description and saw the expected

behaviour.