[tz] Add new timezone for Hanoi Capital, Vietnam

[Guy Harris]

On Feb 17, 2019, at 5:43 PM, Phake Nick <c933103 at gmail.com> wrote:

> It is actually reasonable to have both Asia/Hanoi and Asia/Ho_Chi_Minh in tz database because the two cities (and the two fractions that control them) used different timezone from year 1968 to 1975, which mean even time after the UNIX Epoch would be affected.

Any references for what differed between the two regions?