model-diff: hide content when models are identical

[kyleam]

When passed files with identical contents, diffobj::diffFile() has a message that says "No visible differences between objects" but then confusingly prints a diff with the file content to the console. Catch this case, give a message, and return early.

Closes #516.

[kyleam]

diffobj::diffFile() has a message that says "No visible differences between objects" but then confusingly prints a diff with the file content to the console

Here's an example:

[kyleam]

let's change the message to something like "Relevant model files are identical"

Done. Thanks, @seth127.