Merge branch 'dl/submodule-set-branch'

"git submodule" learns "set-branch" subcommand that allows the submodule.*.branch settings to be modified. * dl/submodule-set-branch: submodule: teach set-branch subcommand submodule--helper: teach config subcommand --unset git-submodule.txt: "--branch <branch>" option defaults to 'master'