Merge branch 'mm/die-with-dashdash-help'

Merge branch 'mm/die-with-dashdash-help'

When the user gives an argument that can be taken as both a revision
name and a pathname without disambiguating with "--", we used to
give a help message "Use '--' to separate".  The message has been
clarified to show where that '--' goes on the command line.

* mm/die-with-dashdash-help:
  setup: clarify error messages for file/revisions ambiguity