Makefile: remove mandatory "spatch" arguments from SPATCH_FLAGS

Makefile: remove mandatory "spatch" arguments from SPATCH_FLAGS

The "--patch ." part of SPATCH_FLAGS added in f57d11728d1 (coccinelle:
put sane filenames into output patches, 2018-07-23) should have been
added unconditionally to the "spatch" invocation instead, using it
isn't optional.

Let's also move the other mandatory flag to come after
$(SPATCH_FLAGS), to ensure that our "--sp-file" overrides any provided
in the environment, both --sp-file <arg> and --patch <arg> are
last-option-wins as far as spatch(1) option parsing is concerned.

The environment variable override was initially added in
a9a884aea57 (coccicheck: use --all-includes by default,
2016-09-30). In practice there's probably nobody that's using
SPATCH_FLAGS to try to intentionally break our invocations, but since
we're changing this let's make it clear what (if anything) we expect
to be overridden by user-supplied flags.

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