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