t4014-format-patch.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/t4014-format-patch.sh

@@ -43,7 +43,7 @@ test_expect_success setup '
 test_expect_success "format-patch --ignore-if-in-upstream" '
 
 	git format-patch --stdout master..side >patch0 &&
-	cnt=`grep "^From " patch0 | wc -l` &&
+	cnt=$(grep "^From " patch0 | wc -l) &&
 	test $cnt = 3
 
 '
@@ -52,7 +52,7 @@ test_expect_success "format-patch --ignore-if-in-upstream" '
 
 	git format-patch --stdout \
 		--ignore-if-in-upstream master..side >patch1 &&
-	cnt=`grep "^From " patch1 | wc -l` &&
+	cnt=$(grep "^From " patch1 | wc -l) &&
 	test $cnt = 2
 
 '
@@ -69,7 +69,7 @@ test_expect_success "format-patch doesn't consider merge commits" '
 	git checkout -b merger master &&
 	test_tick &&
 	git merge --no-ff slave &&
-	cnt=`git format-patch -3 --stdout | grep "^From " | wc -l` &&
+	cnt=$(git format-patch -3 --stdout | grep "^From " | wc -l) &&
 	test $cnt = 3
 '
 
@@ -77,7 +77,7 @@ test_expect_success "format-patch result applies" '
 
 	git checkout -b rebuild-0 master &&
 	git am -3 patch0 &&
-	cnt=`git rev-list master.. | wc -l` &&
+	cnt=$(git rev-list master.. | wc -l) &&
 	test $cnt = 2
 '
 
@@ -85,7 +85,7 @@ test_expect_success "format-patch --ignore-if-in-upstream result applies" '
 
 	git checkout -b rebuild-1 master &&
 	git am -3 patch1 &&
-	cnt=`git rev-list master.. | wc -l` &&
+	cnt=$(git rev-list master.. | wc -l) &&
 	test $cnt = 2
 '