repo: return a namedtuple with full version info

We were returning an e.g. tuple(1,2,3), but that strips off the full
version string which we might want in some places e.g. '1.2.3-rc3'.
Change the return value to a namedtuple so we can pass back up the
full version string.  For code doing a compare with three elements
(all code today), things still work fine as the namedtuple will DTRT
in this scenario.

Bug: https://crbug.com/gerrit/11144
Change-Id: Ib897b5df308116ad1550b0cf18f49afeb662423e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/231053


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