It seems that this somehow got lost in the commit/rebase shuffle and
wasn't caught by the tests that _did_ make it because they were all using
just one file.
As a result of this bug, locals would fail to work correctly in any
configuration with more than one .tf file.
Along with restoring the append/merge behavior, this also reworks some of
the tests to exercise the multi-file case as better insurance against
regressions of this sort in future.
This fixes#15969.
Martin Atkins