doc: refer to tracking configuration as "upstream"

The term "tracking" often creates confusion between remote
tracking branches and local branches which track a remote
branch. The term "upstream" captures more clearly the idea
of "branch A is based on branch B in some way", so it makes
sense to mention it.

At the same time, upstream branches are used for more
than just git-pull these days; let's mention that here.

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