mingw: uglify (a, 0) definitions to shut up warnings

When the result of a (a, 0) expression is not used, MSys2's GCC version
finds it necessary to complain with a warning:

	right-hand operand of comma expression has no effect

Let's just pretend to use the 0 value and have a peaceful and quiet life
again.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/compat/win32/pthread.h b/compat/win32/pthread.h
index d3dd872..20b35a2 100644
--- a/compat/win32/pthread.h
+++ b/compat/win32/pthread.h
@@ -18,7 +18,10 @@
  */
 #define pthread_mutex_t CRITICAL_SECTION
 
-#define pthread_mutex_init(a,b) (InitializeCriticalSection((a)), 0)
+static inline int return_0(int i) {
+	return 0;
+}
+#define pthread_mutex_init(a,b) return_0((InitializeCriticalSection((a)), 0))
 #define pthread_mutex_destroy(a) DeleteCriticalSection((a))
 #define pthread_mutex_lock EnterCriticalSection
 #define pthread_mutex_unlock LeaveCriticalSection