t0014: remove git command upstream of pipe

Before, the `git frotz` command would fail but its return code was
hidden since it was in the upstream of a pipe. Break the pipeline into
two commands so that the return code is no longer lost. Also, mark
`git frotz` with test_must_fail since it's supposed to fail.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0014-alias.sh

@@ -38,8 +38,8 @@ test_expect_success 'looping aliases - internal execution' '
 #'
 
 test_expect_success 'run-command formats empty args properly' '
-    GIT_TRACE=1 git frotz a "" b " " c 2>&1 |
-    sed -ne "/run_command:/s/.*trace: run_command: //p" >actual &&
+    test_must_fail env GIT_TRACE=1 git frotz a "" b " " c 2>actual.raw &&
+    sed -ne "/run_command:/s/.*trace: run_command: //p" actual.raw >actual &&
     echo "git-frotz a '\'''\'' b '\'' '\'' c" >expect &&
     test_cmp expect actual
 '