largefiles: systematic testing of merges to/from largefiles 427ce5633c1c fixed one problem with update and added a test case for it. The test coverage was thus insufficient before that. To make sure we have good test coverage in this area we add systematic testing of all cases of merges that may or may not change normal files to largefiles or vice versa. The tests shows some annoying extra merge prompts in some cases, but these prompts are hard to avoid and they are now "safe" - they do not leave the system in a confused inconsistent state.