t/t5517-push-mirror.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>