t5000-tar-tree.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>

t/t5000-tar-tree.sh

@@ -123,7 +123,7 @@ test_expect_success \
     'add files to repository' \
     'find a -type f | xargs git update-index --add &&
      find a -type l | xargs git update-index --add &&
-     treeid=`git write-tree` &&
+     treeid=$(git write-tree) &&
      echo $treeid >treeid &&
      git update-ref HEAD $(TZ=GMT GIT_COMMITTER_DATE="2005-05-27 22:00:00" \
      git commit-tree $treeid </dev/null)'
@@ -207,15 +207,15 @@ test_expect_success \
 
 test_expect_success 'clients cannot access unreachable commits' '
 	test_commit unreachable &&
-	sha1=`git rev-parse HEAD` &&
+	sha1=$(git rev-parse HEAD) &&
 	git reset --hard HEAD^ &&
 	git archive $sha1 >remote.tar &&
 	test_must_fail git archive --remote=. $sha1 >remote.tar
 '
 
 test_expect_success 'upload-archive can allow unreachable commits' '
 	test_commit unreachable1 &&
-	sha1=`git rev-parse HEAD` &&
+	sha1=$(git rev-parse HEAD) &&
 	git reset --hard HEAD^ &&
 	git archive $sha1 >remote.tar &&
 	test_config uploadarchive.allowUnreachable true &&