submodule--helper: fix a memory leak in "status"

The "status" sub-command was leaking the "struct strvec" it was
setting up for the reasons explained in f92dbdbc6a (revisions API:
don't leak memory on argv elements that need free()-ing, 2022-08-02),
so let's use the "free_removed_argv_elements" option to
setup_revisions() to fix the leak.

Even if we did that, clobbering the "diff_files_args.nr" with the
return value of setup_revisions() would leave leaks in place, but we
can just stop clobbering it.

Ever since that code was added in a9f8a37584 (submodule: port
submodule subcommand 'status' from shell to C, 2017-10-06) we've had
no reason to modify the "nr" member ("argc" at the time): The next use
of "diff_files_args" after this is the "strvec_clear()" at the end of
the function.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

t/t7422-submodule-output.sh

@@ -2,6 +2,7 @@
 
 test_description='submodule --cached, --quiet etc. output'
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-t3100.sh