p5302-pack-index.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/perf/p5302-pack-index.sh

@@ -8,7 +8,7 @@ test_perf_large_repo
 
 test_expect_success 'repack' '
 	git repack -ad &&
-	PACK=`ls .git/objects/pack/*.pack | head -n1` &&
+	PACK=$(ls .git/objects/pack/*.pack | head -n1) &&
 	test -f "$PACK" &&
 	export PACK
 '