workaround old version of /bin/date in msysgit

I don't expect many people other than me to be trying to use msysgit as a build environment, but I found the version of `/bin/date` is pretty ancient there and running `version_git.sh` gives an error ```invalid date `@1406586781'```.

Can someone test this on Mac, make sure using a switch case instead of an if statement doesn't break anything? Works okay on RHEL5, Ubuntu 14.04, Cygwin, MSYS2, and msysgit (wow I'm accumulating a crapton of different build environments).

I don't know the oldest version of git for which `--pretty=format:%ci` works, but that's pretty close to what we want here, so this could be cleaned up and unified across platforms instead of adding a special case for MSYS. `TODO` later?