version: add source versions & User-Agents to the output

Depending on where/how repo is invoked, the active version might be
from a git tree, and it might be different from the .repo/repo.git/
version in the current repo client checkout.  Report both if they're
different so it's clearer.

Lets also include the two different User-Agent's that we set up when
talking to networked services.

Bug: https://crbug.com/gerrit/11144
Change-Id: I2ebb6e3ac30e374a8406cab3e4438087246a8c57
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239234
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>