t/t2102-update-index-symlinks.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>

t/t2102-update-index-symlinks.sh

@@ -23,7 +23,7 @@ git update-index symlink'
 
 test_expect_success \
 'the index entry must still be a symbolic link' '
-case "`git ls-files --stage --cached symlink`" in
+case "$(git ls-files --stage --cached symlink)" in
 120000" "*symlink) echo pass;;
 *) echo fail; git ls-files --stage --cached symlink; (exit 1);;
 esac'