Avoid passing modified SVN revision numbers (of the form 1234M) to

Avoid passing modified SVN revision numbers (of the form 1234M) to
parts of the versioning code which might not like them.

As a result of this checkin, bob builds from modified SVN working
copies will still announce themselves as revision nnnnM in the
textual version strings, but their binary version in the Windows
VERSIONINFO will now be 0.0.0.0.

git-svn-id: http://svn.tartarus.org/sgt/putty@7231 cda61777-01e9-0310-a592-d414129be87e