osutil: treat open modes 'w' and 'a' as 'w+' and 'a+' in posixfile

to work around http://support.microsoft.com/kb/899149.

Also, Microsoft's documentation of the CreateFile Windows API says (quote):

  When an application creates a file across a network, it is better to use
  GENERIC_READ | GENERIC_WRITE for dwDesiredAccess than to use
  GENERIC_WRITE alone. The resulting code is faster, because the
  redirector can use the cache manager and send fewer SMBs with more data.
  This combination also avoids an issue where writing to a file across a
  network can occasionally return ERROR_ACCESS_DENIED.

mercurial/osutil.c

@@ -436,7 +436,14 static PyObject *posixfile(PyObject *sel
 	}
 	else
 		flags = _O_TEXT;
-	if (plus) {
+	if (m0 == 'r' && !plus) {
+		flags |= _O_RDONLY;
+		access = GENERIC_READ;
+	} else {
+		/*
+		work around http://support.microsoft.com/kb/899149 and
+		set _O_RDWR for 'w' and 'a', even if mode has no '+'
+		*/
 		flags |= _O_RDWR;
 		access = GENERIC_READ | GENERIC_WRITE;
 		fpmode[fppos++] = '+';
@@ -446,25 +453,13 static PyObject *posixfile(PyObject *sel
 	switch (m0) {
 	case 'r':
 		creation = OPEN_EXISTING;
-		if (!plus) {
-			flags |= _O_RDONLY;
-			access = GENERIC_READ;
-		}
 		break;
 	case 'w':
 		creation = CREATE_ALWAYS;
-		if (!plus) {
-			access = GENERIC_WRITE;
-			flags |= _O_WRONLY;
-		}
 		break;
 	case 'a':
 		creation = OPEN_ALWAYS;
 		flags |= _O_APPEND;
-		if (!plus) {
-			flags |= _O_WRONLY;
-			access = GENERIC_WRITE;
-		}
 		break;
 	default:
 		PyErr_Format(PyExc_ValueError,