t4008: correct stale comments
A complete rewrite of a single file was originally designed to be
expressed as a deletion immediately followed by a creation of the
same file, and the comments in the test updated here were written to
reflect that design decision made in
f345b0a0 (Add -B flag to diff-*
brothers., 2005-05-30). However, we later realized that a complete
rewrite is merely how a textual diff should be represented at
366175ef (Rework -B output., 2005-06-19), and updated the actual
tests. But we forgot to update the introductory text while doing
so.
Signed-off-by: Junio C Hamano <gitster@pobox.com>