Merge branch 'jk/mergetool'

Cleans up mergetool/difftool combo. * jk/mergetool: mergetools: simplify how we handle "vim" and "defaults" mergetool--lib: don't call "exit" in setup_tool mergetool--lib: improve show_tool_help() output mergetools/vim: remove redundant diff command git-difftool: use git-mergetool--lib for "--tool-help" git-mergetool: don't hardcode 'mergetool' in show_tool_help git-mergetool: remove redundant assignment git-mergetool: move show_tool_help to mergetool--lib