While most version control systems are relatively sane and treat a

filename as a filename, there are a few that don't. We first encountered

this with CVS, which loves to add a ,v to the end of filenames

depending on the method of access. To address this, we added a

normalization method that SCMTools could override.
Impending work on improving support for ClearCase requires even fancier

normalization, and one that requires being able to use some data about

the repository. This change adds the FileDiff extra_data as an argument

to the normalization method.
This is a soft API break. Existing SCMTools which provide a normalize

method that does not take any additional arguments will continue to

function, but emit a deprecation warning.

Ran unit tests.