dirstate: fix compilation warnings in `dirstate_item_set_possibly_dirty()`

Since https://phab.mercurial-scm.org/D11387 (i.e. the same patch as
mentioned in my previous patch), Clang has also started warning about
`dirstate_item_set_possibly_dirty()` missing an explicit return, and
about its use of the result of an assignment as a condition without
using parentheses. This patch fixes that.

Differential Revision: https://phab.mercurial-scm.org/D11445

mercurial/cext/parsers.c

@@ -488,9 +488,8 b' static PyObject *dirstate_item_new_norma'
    to make sure it is correct. */
 static PyObject *dirstate_item_set_possibly_dirty(dirstateItemObject *self)
 {
-	if (self->flags |= dirstate_flag_possibly_dirty) {
-		Py_RETURN_NONE;
-	}
+	self->flags |= dirstate_flag_possibly_dirty;
+	Py_RETURN_NONE;
 }
 
 /* See docstring of the python implementation for details */