t/t3210-pack-refs.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/t3210-pack-refs.sh

@@ -27,7 +27,7 @@ SHA1=
 test_expect_success \
     'see if git show-ref works as expected' \
     'git branch a &&
-     SHA1=`cat .git/refs/heads/a` &&
+     SHA1=$(cat .git/refs/heads/a) &&
      echo "$SHA1 refs/heads/a" >expect &&
      git show-ref a >result &&
      test_cmp expect result'