Add 'z' redaction option to normalise timezone to UTC

[Melusion]

Add z redaction option to normalise timezone to UTC

Closes #39.

What

Adds z to the redaction pattern. When present, author and committer
timestamps are converted to UTC (offset +00:00), removing the timezone
offset as a location fingerprint.

git config privacy.pattern z      # only strip the timezone
git config privacy.pattern mhsz   # full hour + zeroed minutes/seconds, in UTC

Why

git-privacy already redacts when in the day someone commits, but the
timezone offset (+0200, -0700, …) survived untouched. Over a history it
reveals the committer's region — and, via shifts in the offset, relocations and
travel. Issue #39 requested exactly this; the tzcheck hook so far only warns
about timezone changes rather than removing them.

Behaviour

• Applied before the M/d/h/m/s precision reductions, so those operate on
  the resulting UTC wall-clock time. (Example covered by a test: 01:30 +02:00
  → 2018-12-17 23:30 UTC → with h → 00:30 UTC; the date rolling back to the
  17th proves the conversion ran first.)
• With a limit, the working-hours window is therefore interpreted in UTC.
• Naive datetimes are treated as already-UTC and just tagged as such, so the
  result is deterministic regardless of the host's local timezone.
• No change in behaviour when z is absent from the pattern.

Implementation note (open question for maintainers)

z uses datetime.astimezone(UTC) — it preserves the instant and changes
the offset to +00:00 (14:42 +02:00 → 12:42 +00:00). This pairs cleanly with
utils.dt2gitdate, which writes int(d.timestamp()) plus %z.

The alternative would be a relabel-only semantic (keep the wall-clock numbers,
just set the offset to +00:00), which shifts the instant. I went with
astimezone as the natural reading of "redact to UTC", but I'm happy to switch
if you'd prefer the relabel behaviour.

Tests

New TimezoneTestCase in tests/test_timestamp.py covers:

• conversion to UTC,
• instant preservation,
• ordering before the precision reductions,
• the naive-input case.

All existing tests pass unchanged (python -m unittest tests.test_timestamp).

Docs

Help text in gitprivacy.py and both pattern lists in README.md updated to
document the z identifier.