support .gitignore and other git config files in reuse addheader

support .gitignore and other git config files in reuse addheader: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). reuse addheader: error: '.gitignore' does not have a recognised file extension, please use --style