osutil: avoid accidentally destroying the True object in isgui (issue2937)

Needed to use 'Py_RETURN_TRUE' instead of 'return Py_True' to avoid
reference count errors which would randomly crash the Python
executable during merge. This only happened when you had something
configured in merge-tools and the merge was large enough.

mercurial/osutil.c

@@ -523,9 +523,9 static PyObject *isgui(PyObject *self)
 
 	if (dict != NULL) {
 		CFRelease(dict);
-		return Py_True;
+		Py_RETURN_TRUE;
 	} else {
-		return Py_False;
+		Py_RETURN_FALSE;
 	}
 }
 #endif