Merge branch 'ss/submodule-summary-in-c'

Yet another subcommand of "git submodule" is getting rewritten in C. * ss/submodule-summary-in-c: submodule: port submodule subcommand 'summary' from shell to C t7421: introduce a test script for verifying 'summary' output submodule: rename helper functions to avoid ambiguity submodule: remove extra line feeds between callback struct and macro