Go is being proposed as an officially supported language for elements of
OpenStack:

  https://review.openstack.org/#/c/312267/

As such, repos that exist in OpenStack's git infrastructure
are likely to become places from which people might want to go get
things. Allow optional .git suffixes to allow writing code that depends
on git.openstack.org repos that will work with older go versions while
we wait for this support to roll out.

Change-Id: Ia64bdb1dafea33b1c3770803230d30ec1059df22
Reviewed-on: https://go-review.googlesource.com/23135Reviewed-by: Russ Cox <rsc@golang.org>