test-hook.t: don't directly use redirect to /dev/null in hook for Windows

This goes with 57f1dbc99631.  External hooks are run in cmd.exe, which doesn't
know about /dev/null, but sh can handle it.

tests/test-hook.t

@@ -17,7 +17,7 b' commit hooks can see env vars'
   > pretxnclose = sh -c "HG_LOCAL= HG_TAG= python \"$TESTDIR/printenv.py\" pretxnclose"
   > txnclose = sh -c "HG_LOCAL= HG_TAG= python \"$TESTDIR/printenv.py\" txnclose"
   > txnabort = sh -c "HG_LOCAL= HG_TAG= python \"$TESTDIR/printenv.py\" txnabort"
-  > txnclose.checklock = hg debuglock > /dev/null
+  > txnclose.checklock = sh -c "hg debuglock > /dev/null"
   > EOF
   $ echo a > a
   $ hg add a