Merge branch 'jk/tzoffset-fix' into maint

The internal code used to show local timezone offset is not
prepared to handle timestamps beyond year 2100, and gave a
bogus offset value to the caller.  Use a more benign looking
+0000 instead and let "git log" going in such a case, instead
of aborting.

* jk/tzoffset-fix:
  local_tzoffset: detect errors from tm_to_time_t
  t0006: test various date formats
  t0006: rename test-date's "show" to "relative"

diff --cc t/helper/test-date.c
Simple merge