config.mak.dev: add -Wformat-security

We currently build cleanly with -Wformat-security, and it's
a good idea to make sure we continue to do so (since calls
that trigger the warning may be security vulnerabilities).

Note that we cannot use the stronger -Wformat-nonliteral, as
there are case where we are clever with passing around
pointers to string literals. E.g., bisect_rev_setup() takes
bad_format and good_format parameters. These ultimately come
from literals, but they still trigger the warning.

Some of these might be fixable (e.g., by passing flags from
which we locally select a format), and might even be worth
fixing (not because of security, but just because it's an
easy mistake to pass the wrong format). But there are other
cases which are likely quite hard to fix (we actually
generate formats in a local buffer in some cases). So let's
punt on that for now and start with -Wformat-security, which
is supposed to catch the most important cases.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.dev

@@ -2,6 +2,7 @@ ifeq ($(filter no-error,$(DEVOPTS)),)
 CFLAGS += -Werror
 endif
 CFLAGS += -Wdeclaration-after-statement
+CFLAGS += -Wformat-security
 CFLAGS += -Wno-format-zero-length
 CFLAGS += -Wold-style-definition
 CFLAGS += -Woverflow