t/t5510-fetch.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
	perl -i -pe 'BEGIN{undef $/;} s/`(.+?)`/\$(\1)/smg'  "${_f}"
done

and then carefully proof-read.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5510-fetch.sh

@@ -7,7 +7,7 @@ test_description='Per branch config variables affects "git fetch".
 
 . ./test-lib.sh
 
-D=`pwd`
+D=$(pwd)
 
 test_bundle_object_count () {
 	git verify-pack -v "$1" >verify.out &&
@@ -64,8 +64,8 @@ test_expect_success "fetch test" '
 	cd two &&
 	git fetch &&
 	test -f .git/refs/heads/one &&
-	mine=`git rev-parse refs/heads/one` &&
-	his=`cd ../one && git rev-parse refs/heads/master` &&
+	mine=$(git rev-parse refs/heads/one) &&
+	his=$(cd ../one && git rev-parse refs/heads/master) &&
 	test "z$mine" = "z$his"
 '
 
@@ -75,8 +75,8 @@ test_expect_success "fetch test for-merge" '
 	git fetch &&
 	test -f .git/refs/heads/two &&
 	test -f .git/refs/heads/one &&
-	master_in_two=`cd ../two && git rev-parse master` &&
-	one_in_two=`cd ../two && git rev-parse one` &&
+	master_in_two=$(cd ../two && git rev-parse master) &&
+	one_in_two=$(cd ../two && git rev-parse one) &&
 	{
 		echo "$one_in_two	"
 		echo "$master_in_two	not-for-merge"