Merge branch 'ab/push-default-doc-fix' into maint

Doc fix.

* ab/push-default-doc-fix:
  push: mention "push.default=tracking" in the documentation

Author: gitster

Documentation/config.txt

@@ -2438,6 +2438,8 @@ push.default::
   pushing to the same repository you would normally pull from
   (i.e. central workflow).
 
+* `tracking` - This is a deprecated synonym for `upstream`.
+
 * `simple` - in centralized workflow, work like `upstream` with an
   added safety to refuse to push if the upstream branch's name is
   different from the local one.