This matches GNU diff(1) behavior and, more importantly, eliminates any
source of confusion if multiple formatting options are specified.
Note that the committed diff differs slightly from the submitted: I've
modified it so that we initialize diff_format to something that isn't an
accepted format option so that we can also reject --normal -c and -c
--normal, which would've otherwise been accepted because the default was
--normal. After option parsing we default it to D_NORMAL if it's still
unset.
PR: 243975
Submitted by: fehmi noyan isi
MFC after: 1 week
db9cccfbb1