git_command: use subprocess.run for version info

The code is a bit simpler & easier to reason about.

Change-Id: If125ea7d776cdfa38a0440a2b03583de079c4839
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297023


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>