Make git-committer inferred version-date TZ-invariant
Make git-committer inferred version-date TZ-invariant

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

mettekou/ghc,snoyberg/ghc,sgillespie/ghc,oldmanmike/ghc,sgillespie/ghc,GaloisInc/halvm-ghc,vTurbine/ghc,shlevy/ghc,nushio3/ghc,vikraman/ghc,GaloisInc/halvm-ghc,olsner/ghc,mcschroeder/ghc,snoyberg/ghc,GaloisInc/halvm-ghc,sdiehl/ghc,mettekou/ghc,snoyberg/ghc,vikraman/ghc,oldmanmike/ghc,shlevy/ghc,GaloisInc/halvm-ghc,vTurbine/ghc,sgillespie/ghc,sdiehl/ghc,shlevy/ghc,oldmanmike/ghc,ezyang/ghc,mcschroeder/ghc,ezyang/ghc,sdiehl/ghc,nushio3/ghc,gridaphobe/ghc,ezyang/ghc,vikraman/ghc,mcschroeder/ghc,tjakway/ghcjvm,GaloisInc/halvm-ghc,mettekou/ghc,sgillespie/ghc,olsner/ghc,snoyberg/ghc,mettekou/ghc,tjakway/ghcjvm,nushio3/ghc,nushio3/ghc,mettekou/ghc,gridaphobe/ghc,nushio3/ghc,vTurbine/ghc,vTurbine/ghc,vikraman/ghc,mcschroeder/ghc,nushio3/ghc,snoyberg/ghc,mettekou/ghc,vTurbine/ghc,sgillespie/ghc,gridaphobe/ghc,tjakway/ghcjvm,sdiehl/ghc,olsner/ghc,vTurbine/ghc,tjakway/ghcjvm,oldmanmike/ghc,gridaphobe/ghc,mcschroeder/ghc,mcschroeder/ghc,vikraman/ghc,vikraman/ghc,vikraman/ghc,vTurbine/ghc,olsner/ghc,mettekou/ghc,ezyang/ghc,GaloisInc/halvm-ghc,olsner/ghc,shlevy/ghc,shlevy/ghc,oldmanmike/ghc,sdiehl/ghc,gridaphobe/ghc,nushio3/ghc,tjakway/ghcjvm,sdiehl/ghc,sgillespie/ghc,GaloisInc/halvm-ghc,tjakway/ghcjvm,snoyberg/ghc,ezyang/ghc,olsner/ghc,olsner/ghc,oldmanmike/ghc,oldmanmike/ghc,gridaphobe/ghc,shlevy/ghc,tjakway/ghcjvm,mcschroeder/ghc,sgillespie/ghc,gridaphobe/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,snoyberg/ghc,ezyang/ghc