This patch makes several enhancements to the CVS scmtool:

- If a password is given for a pserver, include it in the CVSROOT string.  This makes it so a 'cvs login' is not necessary (though still allowed).

- Tweak the diff parsing to work with patches between multiple revisions, for reviewing existing code.  (The patch will have multiple 'retrieving revision ...' lines.)

- Enhance the error handling to recognize more errors, and to distinguish FileNotFound from more general errors.  For example, a misconfigured repo used to raise FileNotFound, which is confusing.  SCMError makes it more clear.

- We don't have automated tests for pservers, but I manually checked that including the password works.
- Added a new test to make sure that inter-revision diffs parse ok.
- Added a new test to make sure that a bad revision number will raise FileNotFoundError.
- Added a new test to check that a bad Repository config will raise SCMError.