Split a unified diff back into the lines it was assembled from.

Deliberately not splitlines(). Every diff reaching this function is "\n"-joined from lines that themselves came from splitlines() or split("\n"), so no element can contain a line boundary and "\n" is the exact inverse. splitlines() also breaks on \r, \v, \f, U+2028, U+2029 and U+0085, which splits a single diff line into fragments. The tail fragment carries no +/- marker, so it would render as an unmarked note — on the approval prompt that means changed content shown as neutral metadata.

Check any new producer against that invariant rather than against a list of the current ones.