Commit date validation: more stringent checks, more useful error messages.

revlog: raise informative exception if file is missing.

Minor cleanups.

Created a class in util called chunkbuffer that buffers reads from an iterator over strings (aka chunks). Also added to util (for future use) is a generator function that iterates over a file n bytes at a time. Lastly, localrepo was changed to use this new chunkbuffer class when reading changegroups form the local repository.

Add test output.

Date validation must check for 32-bit width. Don't use assert to check.