lib-submodule-update: pass 'test_must_fail' as an argument

When we run a test helper function in test_submodule_switch_common(), we
sometimes specify a whole helper function as the $command. When we do
this, in some test cases, we just mark the whole function with
`test_must_fail`. However, it's possible that the helper function might
fail earlier or later than expected due to an introduced bug. If this
happens, then the test case will still report as passing but it should
really be marked as failing since it didn't actually display the
intended behaviour.

Instead of invoking `test_must_fail $command`, pass the string
"test_must_fail" as the second argument in case where the git command is
expected to fail.

When $command is a helper function, the parent function calling
test_submodule_switch_common() is test_submodule_switch_func(). For all
test_submodule_switch_func() invocations, increase the granularity of
the argument test helper function by prefixing the git invocation which is
meant to fail with the second argument like this:

	$2 git checkout "$1"

In the other cases, test_submodule_switch() and
test_submodule_forced_switch(), instead of passing in the git command
directly, wrap it using the git_test_func() and pass the git arguments
using the global variable $gitcmd. Unfortunately, since closures aren't
a thing in shell scripts, the global variable is necessary. Another
unfortunate result is that the "git_test_func" will used as the test
case name when $command is printed but it's worth it for the cleaner
code.

Finally, as an added bonus, `test_must_fail` will now only run on git
commands.

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

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