Merge branch 'jk/rev-parse-double-dashes' into maint

"git rev-parse <revs> -- <paths>" did not implement the usual
disambiguation rules the commands in the "git log" family used in
the same way.

* jk/rev-parse-double-dashes:
  rev-parse: be more careful with munging arguments
  rev-parse: correctly diagnose revision errors before "--"