C-x 4 a ceased to work in *cvs-diff* buffer

[Kai Grossjohann]

I used to do `=' on a file in a *cvs* buffer, then position point on
a change in the resulting *cvs-diff* buffer, then hit C-x 4 a to
compose a ChangeLog entry.

Now this doesn't find the file anymore and asks me which file is
meant.  It suggests the root directory of the working copy, instead
of the subdir that contains the file.

I first observed this behavior some weeks ago.

(I guess this bug only happens if the file is in a subdir of the
working copy.)

Kai