Add ability to fail loudly if run produces diff

[ZevEisenberg]

Sometimes, a change to an asset catalog gets committed without the corresponding swiftgen command generating source code for it. I have a pre-commit hook, but for whatever reason, sometimes that gets skipped. I don't want to run SwiftGen during every Xcode build for performance reasons.

What I want is to be able to run SwiftGen on my CI server with a --dry-run or --fail-if-diffs mode; something that I can use to fail the build if SwiftGen's output produces any changes. I think the best I can do right now is to run SwiftGen on CI, and then use something like git diff HEAD --exit-code --quiet to see if it produced any changes.

(n.b. git diff HEAD will catch both staged and un-staged files.)