t4006-diff-mode.sh: use the $( ... ) construct for command substitution

The Git CodingGuidelines prefer the $(...) construct for command
substitution instead of using the backquotes `...`.

The backquoted form is the traditional method for command
substitution, and is supported by POSIX.  However, all but the
simplest uses become complicated quickly.  In particular, embedded
command substitutions and/or the use of double quotes require
careful escaping with the backslash character.

The patch was generated by:

for _f in $(find . -name "*.sh")
do
   sed -i 's@`\(.*\)`@$(\1)@g' ${_f}
done

and then carefully proof-read.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4006-diff-mode.sh

@@ -13,7 +13,7 @@ sed_script='s/\(:100644 100755\) \('"$_x40"'\) \2 /\1 X X /'
 test_expect_success 'setup' '
 	echo frotz >rezrov &&
 	git update-index --add rezrov &&
-	tree=`git write-tree` &&
+	tree=$(git write-tree) &&
 	echo $tree
 '