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>
diff --git a/parse-options.c b/parse-options.c
index c1c66bd..67e98a6 100644
--- a/parse-options.c
+++ b/parse-options.c
@@ -18,15 +18,6 @@
 	return error("BUG: switch '%c' %s", opt->short_name, reason);
 }
 
-int opterror(const struct option *opt, const char *reason, int flags)
-{
-	if (flags & OPT_SHORT)
-		return error("switch `%c' %s", opt->short_name, reason);
-	if (flags & OPT_UNSET)
-		return error("option `no-%s' %s", opt->long_name, reason);
-	return error("option `%s' %s", opt->long_name, reason);
-}
-
 static int get_arg(struct parse_opt_ctx_t *p, const struct option *opt,
 		   int flags, const char **arg)
 {
@@ -594,3 +585,12 @@
 	return usage_with_options_internal(ctx, usagestr, opts, 0, err);
 }
 
+#undef opterror
+int opterror(const struct option *opt, const char *reason, int flags)
+{
+	if (flags & OPT_SHORT)
+		return error("switch `%c' %s", opt->short_name, reason);
+	if (flags & OPT_UNSET)
+		return error("option `no-%s' %s", opt->long_name, reason);
+	return error("option `%s' %s", opt->long_name, reason);
+}