push: update remote branch with same name when not providing refspec (no longer require manual configuration of upstream branch)
> When the command line does not specify where to push with the <repository> argument,
> branch.*.remote configuration for the current branch is consulted to determine where to push.
> If the configuration is missing, it defaults to origin.
> When the command line does not specify what to push with <refspec>... arguments or --all, --mirror, --tags options,
> the command finds the default <refspec> by consulting remote.*.push configuration,
> and if it is not found, honors push.default configuration to decide what to push
