t1000-read-tree-m-3way.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: NElia Pinto <gitter.spiros@gmail.com>
Reviewed-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t1000-read-tree-m-3way.sh

@@ -519,10 +519,10 @@ test_expect_success \
     'rm -f .git/index F16 &&
     echo F16 >F16 &&
     git update-index --add F16 &&
-    tree0=`git write-tree` &&
+    tree0=$(git write-tree) &&
     echo E16 >F16 &&
     git update-index F16 &&
-    tree1=`git write-tree` &&
+    tree1=$(git write-tree) &&
     read_tree_must_succeed -m $tree0 $tree1 $tree1 $tree0 &&
     git ls-files --stage'