t/t5900-repo-selection.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/t5900-repo-selection.sh

@@ -15,7 +15,7 @@ make_tree() {
 make_bare() {
 	git init --bare "$1" &&
 	(cd "$1" &&
-	 tree=`git hash-object -w -t tree /dev/null` &&
+	 tree=$(git hash-object -w -t tree /dev/null) &&
 	 commit=$(echo "$1" | git commit-tree $tree) &&
 	 git update-ref HEAD $commit
 	)