branch.autoSetupMerge configuration variable is true.
--set-upstream::
- If specified branch does not exist yet or if `--force` has been
- given, acts exactly like `--track`. Otherwise sets up configuration
- like `--track` would when creating the branch, except that where
- branch points to is not changed.
+ As this option had confusing syntax, it is no longer supported.
+ Please use `--track` or `--set-upstream-to` instead.
-u <upstream>::
--set-upstream-to=<upstream>::