Remove bashism from t3210-pack-refs.sh

This bashism makes the test fail if /bin/sh is not bash.
Signed-off-by: NDennis Stosberg <dennis@stosberg.net>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

t/t3210-pack-refs.sh

@@ -23,7 +23,7 @@ SHA1=
 test_expect_success \
     'see if git show-ref works as expected' \
     'git-branch a &&
-     SHA1=$(< .git/refs/heads/a) &&
+     SHA1=`cat .git/refs/heads/a` &&
      echo "$SHA1 refs/heads/a" >expect &&
      git-show-ref a >result &&
      diff expect result'