Split up "diff_format" into "format" and "line_termination".

Split up "diff_format" into "format" and "line_termination".

This removes the separate "formats" for name and name-with-zero-
termination.

It also removes the difference between HUMAN and MACHINE formats, and
they both become DIFF_FORMAT_RAW, with the difference being just in the
line and inter-filename termination.

It also makes the code easier to understand.