t/t5516-fetch-push.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/t5516-fetch-push.sh

@@ -16,7 +16,7 @@ This test checks the following functionality:
 
 . ./test-lib.sh
 
-D=`pwd`
+D=$(pwd)
 
 mk_empty () {
 	repo_name="$1"
@@ -422,7 +422,7 @@ test_expect_success 'push tag with non-existent, incomplete dest' '
 test_expect_success 'push sha1 with non-existent, incomplete dest' '
 
 	mk_test testrepo &&
-	test_must_fail git push testrepo `git rev-parse master`:foo
+	test_must_fail git push testrepo $(git rev-parse master):foo
 
 '