silence some -Wuninitialized false positives

There are a few error functions that simply wrap error() and
provide a standardized message text. Like error(), they
always return -1; knowing that can help the compiler silence
some false positive -Wuninitialized warnings.

One strategy would be to just declare these as inline in the
header file so that the compiler can see that they always
return -1. However, gcc does not always inline them (e.g.,
it will not inline opterror, even with -O3), which renders
our change pointless.

Instead, let's follow the same route we did with error() in
the last patch, and define a macro that makes the constant
return value obvious to the compiler.

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

config.c

@@ -1660,6 +1660,7 @@ int git_config_rename_section(const char *old_name, const char *new_name)
  * Call this to report error for your variable that should not
  * get a boolean value (i.e. "[my] var" means "true").
  */
+#undef config_error_nonbool
 int config_error_nonbool(const char *var)
 {
 	return error("Missing value for '%s'", var);