As there is no portable way to pass timezone information to
strftime, some output format from "git log" and friends are
impossible to produce.  Teach our own strbuf_addftime to replace %z
and %Z with caller-supplied values to help working around this.

* rs/strbuf-addftime-zZ:
  date: use localtime() for "-local" time formats
  t0006: check --date=format zone offsets
  strbuf: let strbuf_addftime handle %z and %Z itself