version: add git_user_agent function

version: add git_user_agent function

This is basically a fancy way of saying "git/$GIT_VERSION",
except that it is overridable at build-time and through the
environment. Which means that people who don't want to
advertise their git version (for privacy or security
reasons) can tweak it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>