getdate.y: do not ignore TZ with relative day, month or year offset

getdate.y: do not ignore TZ with relative day, month or year offset

* lib/getdate.y (get_date): Move the tz-handling block to follow the
relative-date-handling, since otherwise, the latter would clobber the
sole output (an updated Start value) of the tz-handling block.
* tests/test-getdate.c: Tests for the fix