move UserAgent to git_command for wider user

We can't import the main module, so move the UserAgent helper out of
it and into the git_command module so it can be used in more places.

Bug: https://crbug.com/gerrit/11144
Change-Id: I8093c8a20bd1dc7d612d0e2a85180341817c0d86
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/231057
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>