Log Message:
Apply the latest bugfixes to the CVS backend from my git repo:

* `6753235d`: Return bounded output from `rcs_diff()` when asked, as
  the API states.
* `e45175d5`: Always explicitly set CVS keyword substitution behavior.
  Fixes behavior when a text file is added under a name formerly
  used for a binary file.
* `b30cacdf`: If the previous working directory no longer exists after
  a CVS operation, don't try to `chdir()` back to it afterward.

Bump PKGREVISION.