Merge branch 'mm/die-with-dashdash-help' into maint-1.7.11

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