diff: don't force the format to 'context' with -p immediately

Instead, leave the fomat as unspecified (if it hasn't been) and use the
-p flag as a hint to 'context' if no other formatting option is specified.

This fixes diff -purw, used frequently by emaste, and matches the behavior
of its GNU counterpart.

PR: 250015
Reviewed by: emaste
MFC after: 1 week