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

diff --git a/t/t4006-diff-mode.sh b/t/t4006-diff-mode.sh
index 05911492ca6df386bfe344ffcfb9562beeaceabf..76f643b2c2a2c7e307135de6942cf46743b6cbef 100755 (executable)
--- a/t/t4006-diff-mode.sh
+++ b/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
 '