contrib/examples/git-repack.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>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/examples/git-repack.sh

@@ -67,8 +67,8 @@ case ",$all_into_one," in
 ,t,)
 	args= existing=
 	if [ -d "$PACKDIR" ]; then
-		for e in `cd "$PACKDIR" && find . -type f -name '*.pack' \
-			| sed -e 's/^\.\///' -e 's/\.pack$//'`
+		for e in $(cd "$PACKDIR" && find . -type f -name '*.pack' \
+			| sed -e 's/^\.\///' -e 's/\.pack$//')
 		do
 			if [ -e "$PACKDIR/$e.keep" ]; then
 				: keep