mingw: use _commit to implement fsync

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -80,7 +80,7 @@ static inline int fork(void)
 static inline unsigned int alarm(unsigned int seconds)
 { return 0; }
 static inline int fsync(int fd)
-{ return 0; }
+{ return _commit(fd); }
 static inline int getppid(void)
 { return 1; }
 static inline void sync(void)