Handle alternate null filename for cvs.
Handle alternate null filename for cvs.

'cvs diff' usually uses '/dev/null' for the name of old files, but on Windows
it can return 'nul:'. Check for this as well. Trivial change.

Fixes bug 2294.

beol/reviewboard,KnowNo/reviewboard,1tush/reviewboard,KnowNo/reviewboard,chipx86/reviewboard,davidt/reviewboard,chipx86/reviewboard,beol/reviewboard,custode/reviewboard,chipx86/reviewboard,bkochendorfer/reviewboard,brennie/reviewboard,chipx86/reviewboard,beol/reviewboard,reviewboard/reviewboard,custode/reviewboard,sgallagher/reviewboard,beol/reviewboard,brennie/reviewboard,brennie/reviewboard,KnowNo/reviewboard,1tush/reviewboard,bkochendorfer/reviewboard,reviewboard/reviewboard,1tush/reviewboard,KnowNo/reviewboard,1tush/reviewboard,davidt/reviewboard,sgallagher/reviewboard,davidt/reviewboard,custode/reviewboard,davidt/reviewboard,custode/reviewboard,1tush/reviewboard,reviewboard/reviewboard,bkochendorfer/reviewboard,1tush/reviewboard,sgallagher/reviewboard,bkochendorfer/reviewboard,1tush/reviewboard,sgallagher/reviewboard,brennie/reviewboard,1tush/reviewboard,reviewboard/reviewboard,1tush/reviewboard