getdate: correctly interpret "next monday" when run on a Monday

getdate: correctly interpret "next monday" when run on a Monday

* lib/getdate.y (get_date): Correct the calculation of tm_mday so
that e.g., "next tues" (when run on a tuesday) results in a date
that is one week in the future, and not today's date.
I.e., add a week when the wday is the same as the current one.
Reported by Tom Broadhurst in http://savannah.gnu.org/bugs/?25406,
and earlier by Martin Bernreuther and Jan Minář.
* tests/test-getdate.c (main): Check that "next DAY" is always in
the future and that "last DAY" is always in the past.