Makefile: add a DEVOPTS to get all of -Wextra

Change DEVOPTS to understand a "extra-all" option. When the DEVELOPER
flag is enabled we turn on -Wextra, but manually switch some of the
warnings it turns on off.

This is because we have many existing occurrences of them in the code
base. This mode will stop the suppression, let the developer see and
decide whether to  fix them.

This change is a slight alteration of Nguyễn Thái Ngọc Duy
EAGER_DEVELOPER mode patch[1]

1. "[PATCH v3 3/3] Makefile: add EAGER_DEVELOPER
    mode" (<20180329150322.10722-4-pclouds@gmail.com>;
    https://public-inbox.org/git/20180329150322.10722-4-pclouds@gmail.com/)

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

Makefile

@@ -445,6 +445,12 @@ all::
 #        suppresses the -Werror that implicitly comes with
 #        DEVELOPER=1. Useful for getting the full set of errors
 #        without immediately dying, or for logging them.
+#
+#    extra-all:
+#
+#        The DEVELOPER mode enables -Wextra with a few exceptions. By
+#        setting this flag the exceptions are removed, and all of
+#        -Wextra is used.
 
 GIT-VERSION-FILE: FORCE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN

config.mak.dev

@@ -23,6 +23,7 @@ CFLAGS += -Wextra
 # if a function is public, there should be a prototype and the right
 # header file should be included. If not, it should be static.
 CFLAGS += -Wmissing-prototypes
+ifeq ($(filter extra-all,$(DEVOPTS)),)
 # These are disabled because we have these all over the place.
 CFLAGS += -Wno-empty-body
 CFLAGS += -Wno-missing-field-initializers
@@ -30,6 +31,7 @@ CFLAGS += -Wno-sign-compare
 CFLAGS += -Wno-unused-function
 CFLAGS += -Wno-unused-parameter
 endif
+endif
 
 # uninitialized warnings on gcc 4.9.2 in xdiff/xdiffi.c and config.c
 # not worth fixing since newer compilers correctly stop complaining