t4116-apply-reverse.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/t4116-apply-reverse.sh

@@ -30,10 +30,10 @@ test_expect_success setup '
 
 test_expect_success 'apply in forward' '
 
-	T0=`git rev-parse "second^{tree}"` &&
+	T0=$(git rev-parse "second^{tree}") &&
 	git reset --hard initial &&
 	git apply --index --binary patch &&
-	T1=`git write-tree` &&
+	T1=$(git write-tree) &&
 	test "$T0" = "$T1"
 '
 
@@ -62,22 +62,22 @@ test_expect_success 'setup separate repository lacking postimage' '
 
 test_expect_success 'apply in forward without postimage' '
 
-	T0=`git rev-parse "second^{tree}"` &&
+	T0=$(git rev-parse "second^{tree}") &&
 	(
 		cd initial &&
 		git apply --index --binary ../patch &&
-		T1=`git write-tree` &&
+		T1=$(git write-tree) &&
 		test "$T0" = "$T1"
 	)
 '
 
 test_expect_success 'apply in reverse without postimage' '
 
-	T0=`git rev-parse "initial^{tree}"` &&
+	T0=$(git rev-parse "initial^{tree}") &&
 	(
 		cd second &&
 		git apply --index --binary --reverse ../patch &&
-		T1=`git write-tree` &&
+		T1=$(git write-tree) &&
 		test "$T0" = "$T1"
 	)
 '