t/t5506-remote-groups.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/t5506-remote-groups.sh

@@ -20,7 +20,7 @@ update_repos() {
 }
 
 repo_fetched() {
-	if test "`git log -1 --pretty=format:%s $1 --`" = "`cat mark`"; then
+	if test "$(git log -1 --pretty=format:%s $1 --)" = "$(cat mark)"; then
 		echo >&2 "repo was fetched: $1"
 		return 0
 	fi