Fix --diff output when encountering EOF

[akien-mga]

split("\n") includes a final empty element "" if the final line
ends with \n (as it should for POSIX-compliant text files), which
then became an extra "\n".

splitlines() solves that, but there's a caveat, as it will split
on other types of line breaks too (like \r), which may not be
desired.

Fixes #526.

Thanks to @benkuhn for the well-debugged issue.

[akien-mga]

As mentioned, splitlines() works well but it might be doing more than wanted here. I'm not a Python guru, but I'm sure someone may have an elegant idea for ensuring that we can convert a string with \n to a list of lines with trailing \n, without having this extra empty line.

[akien-mga]

Fixed the test which was also exhibiting this bug (and thus failing after the fix).

[akien-mga]

Note that I didn't touch other uses of split("\n") which are not linked to the referenced issue, but contributors here might want to check if those occurrences can trigger a similar EOF issue:

$ rg --fixed-strings 'split("\n"'
black.py
2379:    for index, line in enumerate(prefix.split("\n")):

tests/data/comments2.py
299:        element.split("\n", 1)[0]

tests/data/comments3.py
10:        element.split("\n", 1)[0]

[JelleZijlstra]

Thanks! The other cases look harmless (two are in test data and one always ignores an empty line at the end).