push: document the future default change for push.default (matching -> simple)

It is too early to start warning loudly about the future default change
in favor of 'simple', since many users use different versions of Git, and
would be harmed if we advised them to explicitely set
'push.default=simple' when using old versions of Git.

Still, we want to document the upcomming change so that:

* Users who may be affected by the change get one more chance to know it
  in advance.

* We actually commit to changing the default, and avoid repeating past
  errors.

Signed-off-by: Matthieu Moy <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: moy

Documentation/config.txt

@@ -1685,10 +1685,14 @@ push.default::
   shape and then push them out with a single command.  It is not
   appropriate for pushing into a repository shared by multiple users,
   since locally stalled branches will attempt a non-fast forward push
-  if other users updated the branch.  This is the default.
+  if other users updated the branch.
+  +
+  This is currently the default, but Git 2.0 will change the default
+  to `simple`.
 * `simple` - like `upstream`, but refuses to push if the upstream
   branch's name is different from the local one. This is the safest
-  option and is well-suited for beginners.
+  option and is well-suited for beginners. It will become the default
+  in Git 2.0.
 * `upstream` - push the current branch to its upstream branch.
   With this, `git push` will update the same remote ref as the one which
   is merged by `git pull`, making `push` and `pull` symmetrical.