ls-tree manpage: use "unless" instead of "when ... is not"

Delayed negation in a statement is harder to spot and keep in mind.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-ls-tree.txt

@@ -76,7 +76,7 @@ Output Format
 -------------
         <mode> SP <type> SP <object> TAB <file>
 
-When the `-z` option is not used, TAB, LF, and backslash characters
+Unless the `-z` option is used, TAB, LF, and backslash characters
 in pathnames are represented as `\t`, `\n`, and `\\`, respectively.
 
 When the `-l` option is used, format changes to