test-lib-functions: remove bug-inducing "diagnostics" helper param

test-lib-functions: remove bug-inducing "diagnostics" helper param

Remove the optional "diagnostics" parameter of the
test_path_is_{file,dir,missing} functions.

We have a lot of uses of these functions, but the only legitimate use
of the diagnostics parameter is from when the functions themselves
were introduced in 2caf20c52b7 (test-lib: user-friendly alternatives
to test [-d|-f|-e], 2010-08-10).

But as the the rest of this diff demonstrates its presence did more to
silently introduce bugs in our tests. Fix such bugs in the tests added
in ae4e89e549b (gc: add --keep-largest-pack option, 2018-04-15), and
c04ba51739a (t6046: testcases checking whether updates can be skipped
in a merge, 2018-04-19).

Let's also assert that those functions are called with exactly one
parameter, a follow-up commit will add similar asserts to other
functions in test-lib-functions.sh that we didn't have existing misuse
of.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>