tools/build_version_flags.sh: Use shorter Git hashes.
tools/build_version_flags.sh: Use shorter Git hashes.

This will make the --version more readable.

before:

$ vtworker --version
Version: 512ba511699515567e7326b5fb33265eb38a55f6 built on Wed Jun 14 22:30:38 PDT 2017 by mberlin@hostname using go1.8 linux/amd64

after:

$ vtworker --version
Version: a10ad948eca957ff9aab544dcdf950d893f11e21@hostname using go1.8 linux/amd64

alainjobart/vitess,HubSpot/vitess,mahak/vitess,mahak/vitess,davygeek/vitess,rnavarro/vitess,enisoc/vitess,mattharden/vitess,mattharden/vitess,vitessio/vitess,sougou/vitess,dcadevil/vitess,vitessio/vitess,rnavarro/vitess,rnavarro/vitess,rnavarro/vitess,pivanof/vitess,rnavarro/vitess,tinyspeck/vitess,pivanof/vitess,mattharden/vitess,HubSpot/vitess,mattharden/vitess,enisoc/vitess,davygeek/vitess,pivanof/vitess,vitessio/vitess,alainjobart/vitess,dcadevil/vitess,enisoc/vitess,dcadevil/vitess,HubSpot/vitess,mahak/vitess,dcadevil/vitess,mattharden/vitess,HubSpot/vitess,tirsen/vitess,alainjobart/vitess,sougou/vitess,tinyspeck/vitess,mattharden/vitess,vitessio/vitess,HubSpot/vitess,mattharden/vitess,sougou/vitess,mahak/vitess,davygeek/vitess,sougou/vitess,HubSpot/vitess,pivanof/vitess,sougou/vitess,rnavarro/vitess,tirsen/vitess,pivanof/vitess,mahak/vitess,alainjobart/vitess,alainjobart/vitess,tirsen/vitess,tirsen/vitess,davygeek/vitess,enisoc/vitess,tinyspeck/vitess,vitessio/vitess,HubSpot/vitess,pivanof/vitess,davygeek/vitess,dcadevil/vitess,enisoc/vitess,pivanof/vitess,tirsen/vitess,tinyspeck/vitess,vitessio/vitess,enisoc/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,davygeek/vitess,rnavarro/vitess,tirsen/vitess,sougou/vitess,dcadevil/vitess,alainjobart/vitess,HubSpot/vitess,tirsen/vitess,rnavarro/vitess,davygeek/vitess,alainjobart/vitess,tirsen/vitess,sougou/vitess,tinyspeck/vitess,tinyspeck/vitess,mattharden/vitess,tinyspeck/vitess,alainjobart/vitess,mahak/vitess,enisoc/vitess,tirsen/vitess,sougou/vitess,mattharden/vitess,pivanof/vitess,dcadevil/vitess,pivanof/vitess,rnavarro/vitess,mahak/vitess,mattharden/vitess