t4010-diff-pathspec.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/t4010-diff-pathspec.sh

@@ -18,7 +18,7 @@ test_expect_success \
      mkdir path1 &&
      echo rezrov >path1/file1 &&
      git update-index --add file0 path1/file1 &&
-     tree=`git write-tree` &&
+     tree=$(git write-tree) &&
      echo "$tree" &&
      echo nitfol >file0 &&
      echo yomin >path1/file1 &&
@@ -131,7 +131,7 @@ test_expect_success 'diff multiple wildcard pathspecs' '
 	mkdir path2 &&
 	echo rezrov >path2/file1 &&
 	git update-index --add path2/file1 &&
-	tree3=`git write-tree` &&
+	tree3=$(git write-tree) &&
 	git diff --name-only $tree $tree3 -- "path2*1" "path1*1" >actual &&
 	cat <<-\EOF >expect &&
 	path1/file1