git_command: drop custom version helper

Since ParseGitVersion can call `git --version` automatically, we don't
need this duplicate version() helper anymore.  The only other user is
the `repo version` code, so convert that to version_tuple().full.

Bug: https://crbug.com/gerrit/11144
Change-Id: I9d77822fc39f4ba28884d9183359169cabf5f17d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/231055


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