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.h b/parse-options.h
index 71a39c6..e703853 100644
--- a/parse-options.h
+++ b/parse-options.h
@@ -177,6 +177,10 @@
 
 extern int optbug(const struct option *opt, const char *reason);
 extern int opterror(const struct option *opt, const char *reason, int flags);
+#ifdef __GNUC__
+#define opterror(o,r,f) (opterror((o),(r),(f)), -1)
+#endif
+
 /*----- incremental advanced APIs -----*/
 
 enum {