The existing code suffers from the issue of converting the
committer date to localtime, thereby resulting in varying
inferred dates for the same commit-hash depending on the currently
set local timezone.

In order to have a universally unique mapping between commit-hashes and
their snapshot-version-date it's better to convert to the date expressed
in a fixed timezone like e.g. UTC.

Sadly, `git` doesn't seem to provide a way to directly format dates the
way we need it, so we shell out to `perl` for this.

Reviewers: austin, thomie, erikd, bgamari

Differential Revision: https://phabricator.haskell.org/D1711