t7900-subtree.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>