t/t5532-fetch-proxy.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/t5532-fetch-proxy.sh

@@ -15,7 +15,7 @@ test_expect_success 'setup remote repo' '
 cat >proxy <<'EOF'
 #!/bin/sh
 echo >&2 "proxying for $*"
-cmd=`"$PERL_PATH" -e '
+cmd=$("$PERL_PATH" -e '
 	read(STDIN, $buf, 4);
 	my $n = hex($buf) - 4;
 	read(STDIN, $buf, $n);
@@ -23,7 +23,7 @@ cmd=`"$PERL_PATH" -e '
 	# drop absolute-path on repo name
 	$cmd =~ s{ /}{ };
 	print $cmd;
-'`
+')
 echo >&2 "Running '$cmd'"
 exec $cmd
 EOF