push.default: Rename 'tracking' to 'upstream'

push.default: Rename 'tracking' to 'upstream'

Users are sometimes confused with two different types of "tracking" behavior
in Git: "remote-tracking" branches (e.g. refs/remotes/*/*) versus the
merge/rebase relationship between a local branch and its @{upstream}
(controlled by branch.foo.remote and branch.foo.merge config settings).

When the push.default is set to 'tracking', it specifies that a branch should
be pushed to its @{upstream} branch. In other words, setting push.default to
'tracking' applies only to the latter of the above two types of "tracking"
behavior.

In order to make this more understandable to the user, we rename the
push.default == 'tracking' option to push.default == 'upstream'.

push.default == 'tracking' is left as a deprecated synonym for 'upstream'.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>