Merge branch 'da/difftool-mergetool-simplify-reporting-status'

Code simplification.

* da/difftool-mergetool-simplify-reporting-status:
  mergetools: stop setting $status in merge_cmd()
  mergetool: simplify conditionals
  difftool--helper: add explicit exit statement
  mergetool--lib: remove use of $status global
  mergetool--lib: remove no-op assignment to $status from setup_user_tool