compiletest: investigate if compare-mode nits are still relevant

also, I don't know if this is exactly relevant to this particular PR, but during my experimentation: When I created an empty augmented-assignments.nll.stderr file, I expected to see a diff that just showed all plusses. (I.e., a diff against an empty file)

But instead it seems like if the revision file is empty, then it falls back on the base file augmented-assignments.stderr, in this particular case...

• (From skimming the PR, I'm not even clear on how we are observing such a semantics... but nonetheless, it is what I am seeing.)

(I don't know whether other people think that fall back behavior for empty files is more intutive... it wasn't intuitive for me, since I'm the kind of person who will just do touch augmented-assignments.nll.stderr and then run the tool to see what the content is that I need to add.)

Originally posted by @pnkfelix in https://github.com/rust-lang/rust/issues/49293#issuecomment-379049906

Start by locating compiletest's compare-mode handling and the augmented-assignments.nll.stderr and augmented-assignments.stderr test files. Reproduce the behavior with an empty revision file, then determine the intended result and capture the expected behavior in the relevant test or issue documentation.