date: allow multiple --reference

[cakebaker]

This PR allows multiple uses of --reference to match the behavior of GNU date. The last reference "wins".

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/expand/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.