t/t3100-ls-tree-restrict.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/t3100-ls-tree-restrict.sh

@@ -28,7 +28,7 @@ test_expect_success \
      echo Mi >path2/baz/b &&
      find path? \( -type f -o -type l \) -print |
      xargs git update-index --add &&
-     tree=`git write-tree` &&
+     tree=$(git write-tree) &&
      echo $tree'
 
 test_output () {