test-lib: make test_expect_code a test command

Change test_expect_code to be a normal test command instead of a
top-level command.

As a top-level command it would fail in cases like:

    test_expect_code 1 'phoney' '
        foo && bar && (exit 1)
    '

Here the test might incorrectly succeed if "foo" or "bar" happened to
fail with exit status 1. Instead we now do:

    test_expect_success 'phoney' '
        foo && bar && test_expect_code 1 "(exit 1)"
    '

Which will only succeed if "foo" and "bar" return status 0, and "(exit
1)" returns status 1.  Note that test_expect_code has been made slightly
noisier, as it reports the exit code it receives even upon success.

Some test code in t0000-basic.sh relied on the old semantics of
test_expect_code to test the test_when_finished command. I've
converted that code to use an external test similar to the TODO test I
added in v1.7.3-rc0~2^2~3.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/README

@@ -395,13 +395,6 @@ library for your script to use.
    Like test_expect_success this function can optionally use a three
    argument invocation with a prerequisite as the first argument.
 
- - test_expect_code [<prereq>] <code> <message> <script>
-
-   Analogous to test_expect_success, but pass the test if it exits
-   with a given exit <code>
-
- test_expect_code 1 'Merge with d/f conflicts' 'git merge "merge msg" B master'
-
  - test_debug <script>
 
    This takes a single argument, <script>, and evaluates it only
@@ -482,6 +475,15 @@ library for your script to use.
 	    'Perl API' \
 	    "$PERL_PATH" "$TEST_DIRECTORY"/t9700/test.pl
 
+ - test_expect_code <exit-code> <command>
+
+   Run a command and ensure that it exits with the given exit code.
+   For example:
+
+	test_expect_success 'Merge with d/f conflicts' '
+		test_expect_code 1 git merge "merge msg" B master
+	'
+
  - test_must_fail <git-command>
 
    Run a git command and ensure it fails in a controlled way.  Use