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>

t/t4008-diff-break-rewrite.sh

@@ -10,14 +10,14 @@ We have two very different files, file0 and file1, registered in a tree.
 We update file1 so drastically that it is more similar to file0, and
 then remove file0.  With -B, changes to file1 should be broken into
 separate delete and create, resulting in removal of file0, removal of
-original file1 and creation of completely rewritten file1.
+original file1 and creation of completely rewritten file1.  The latter
+two are then merged back into a single "complete rewrite".
 
 Further, with -B and -M together, these three modifications should
 turn into rename-edit of file0 into file1.
 
 Starting from the same two files in the tree, we swap file0 and file1.
-With -B, this should be detected as two complete rewrites, resulting in
-four changes in total.
+With -B, this should be detected as two complete rewrites.
 
 Further, with -B and -M together, these should turn into two renames.
 '