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