PersonIdent: Preserve the timezone when copying with new time

The PersonIdent(PersonIdent,Date) constructor must create a copy with
the same author/email/timezone but different time.  When we changed
the implementation to the new Instant/ZoneId version, we forgot to
pass the timezone. This made fail some tests downstream.

Pass the timezone when constructing the copy.

Change-Id: Iaa979e6dbaa3c55d4c4d2040068ab8b03163cd4e