inline constant return from error() function

Commit e208f9cc introduced a macro to turn error() calls
into:

  (error(), -1)

to make the constant return value more visible to the
calling code (and thus let the compiler make better
decisions about the code).

This works well for code like:

  return error(...);

but the "-1" is superfluous in code that just calls error()
without caring about the return value. In older versions of
gcc, that was fine, but gcc 4.9 complains with -Wunused-value.

We can work around this by encapsulating the constant return
value in a static inline function, as gcc specifically
avoids complaining about unused function returns unless the
function has been specifically marked with the
warn_unused_result attribute.

We also use the same trick for config_error_nonbool and
opterror, which learned the same error technique in a469a101.
Reported-by: NFelipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -1184,7 +1184,7 @@ extern int git_env_bool(const char *, int);
 extern int git_config_system(void);
 extern int config_error_nonbool(const char *);
 #if defined(__GNUC__) && ! defined(__clang__)
-#define config_error_nonbool(s) (config_error_nonbool(s), -1)
+#define config_error_nonbool(s) (config_error_nonbool(s), const_error())
 #endif
 extern const char *get_log_output_encoding(void);
 extern const char *get_commit_output_encoding(void);

git-compat-util.h

@@ -343,7 +343,11 @@ extern void warning(const char *err, ...) __attribute__((format (printf, 1, 2)))
  * using the function as usual.
  */
 #if defined(__GNUC__) && ! defined(__clang__)
-#define error(...) (error(__VA_ARGS__), -1)
+static inline int const_error(void)
+{
+	return -1;
+}
+#define error(...) (error(__VA_ARGS__), const_error())
 #endif
 
 extern void set_die_routine(NORETURN_PTR void (*routine)(const char *err, va_list params));

parse-options.h

@@ -180,7 +180,7 @@ extern NORETURN void usage_msg_opt(const char *msg,
 extern int optbug(const struct option *opt, const char *reason);
 extern int opterror(const struct option *opt, const char *reason, int flags);
 #if defined(__GNUC__) && ! defined(__clang__)
-#define opterror(o,r,f) (opterror((o),(r),(f)), -1)
+#define opterror(o,r,f) (opterror((o),(r),(f)), const_error())
 #endif
 
 /*----- incremental advanced APIs -----*/