`unused-revision-names` compiletest directive has confusing name

[lolbinarycat]

In rust, and in programming in general, there are two similar classes of errors you can make with names:

• undefined/unknown/unresolved: trying to refer to something that has is not defined. this corresponds to E0425 in rust.
• unused: defining something, and then not using it. this corresponds to the unused lints in rustc.

despite it's name, unused-revision-names controls how to treat references to revision names that have not been declared. revision names that are defined and not referenced are never an error.

while there is some sense in which these revisions are never "used" (i.e. the test is never run with that revision), this seems needlessly confusing and contrary to the typical meaning of these words.