Unicode strings treat form feed characters as newlines when calling

splitlines(). This behavior doesn't match byte strings, and definitely

doesn't match patch's logic. Form feed characters, from patch and diff's

point of view, are valid characters like any other, and only \r and

\n should be used as line endings.
We now have our own string splitting function that looks for the

patterns of newlines that we and patch support. This gives us more

specific, testable behavior that won't vary based on the underlying

string type or encoding.

Added a unit test, which passes (along with all other tests).
Tested with a diff containing a form feed character. Previously, it would

turn into a newline, corrupting the diff and angering patch. After, the

character was preserved and came along with the patch.
Went through nearly all my local review requests containing diffs. None of them

broke.