Skip Diff if too many files

[pollockm]

Diff tooling becomes useless if there are 100s of files to diff - I don't expect any user would actually wade through the generated results.

Diff tooling also takes a very long time to execute if many many files need to be diffed, leading to issues like #55 and #54

Suggest that we add a check for how many files need to be diffed, add a command-line input for the threshhold, and skip the diff if too many files need to be diffed.