Merge branch 'rs/strbuf-addftime-zZ'

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

Documentation/rev-list-options.txt

@@ -769,7 +769,8 @@ timezone value.
 1970).  As with `--raw`, this is always in UTC and therefore `-local`
 has no effect.
 +
-`--date=format:...` feeds the format `...` to your system `strftime`.
+`--date=format:...` feeds the format `...` to your system `strftime`,
+except for %z and %Z, which are handled internally.
 Use `--date=format:%c` to show the date in your system locale's
 preferred format.  See the `strftime` manual for a complete list of
 format placeholders. When using `-local`, the correct syntax is