* Test that adding a text file under a name formerly tracked as
binary (and vice versa) gets the right keyword-substitution
behavior.
* Explicitly set -kkv for text files to make the tests pass.
* CVS warns in these cases about "changing keyword expansion mode",
but this is correct behavior, so filter it from stderr. Filter
stdout the same way in case we ever want to keep any of it.
* In rcs_add(), replace comments with obviousness.