repo: all ParseGitVersion to load git version info itself

All code that calls ParseGitVersion needs to run `git --version`
itself and parse the output before passing it in.  To avoid that
duplication, allow ParseGitVersion to run `git --version` itself
if ver_str=None.

Bug: https://crbug.com/gerrit/11144
Change-Id: Ie07793ca57a40c0231af808df04a576118d5eea3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/231054


Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>