Fix possible performance problems with WatchDog

[Inventitech]

WatchDog currently analyzes all files, no matter how big. The computations on such files (wrt. diff and static analysis warnings) become at least quadratically more expensive in file size.

However, most source code files are pretty small. It would thus be pretty exceptional to encounter a long source code file. Make WatchDog only operate on small-ish files to fix possible performance problems.