t1002-read-tree-m-u-2way.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/t1002-read-tree-m-u-2way.sh

@@ -21,7 +21,7 @@ compare_change () {
 }
 
 check_cache_at () {
-	clean_if_empty=`git diff-files -- "$1"`
+	clean_if_empty=$(git diff-files -- "$1")
 	case "$clean_if_empty" in
 	'')  echo "$1: clean" ;;
 	?*)  echo "$1: dirty" ;;
@@ -41,14 +41,14 @@ test_expect_success \
      echo bozbar >bozbar &&
      echo rezrov >rezrov &&
      git update-index --add nitfol bozbar rezrov &&
-     treeH=`git write-tree` &&
+     treeH=$(git write-tree) &&
      echo treeH $treeH &&
      git ls-tree $treeH &&
 
      echo gnusto >bozbar &&
      git update-index --add frotz bozbar --force-remove rezrov &&
      git ls-files --stage >M.out &&
-     treeM=`git write-tree` &&
+     treeM=$(git write-tree) &&
      echo treeM $treeM &&
      git ls-tree $treeM &&
      sum bozbar frotz nitfol >M.sum &&
@@ -318,7 +318,7 @@ test_expect_success \
     'rm -f .git/index &&
      echo DF >DF &&
      git update-index --add DF &&
-     treeDF=`git write-tree` &&
+     treeDF=$(git write-tree) &&
      echo treeDF $treeDF &&
      git ls-tree $treeDF &&
 
@@ -326,7 +326,7 @@ test_expect_success \
      mkdir DF &&
      echo DF/DF >DF/DF &&
      git update-index --add --remove DF DF/DF &&
-     treeDFDF=`git write-tree` &&
+     treeDFDF=$(git write-tree) &&
      echo treeDFDF $treeDFDF &&
      git ls-tree $treeDFDF &&
      git ls-files --stage >DFDF.out'