parsers: fix memory leak in compute_phases_map_sets

PySet_Add increments the reference of the added object to the set, see:
https://hg.python.org/cpython/file/2.6/Objects/setobject.c#l379
Before this patch we were forgetting to decrement the reference count after
adding objects to the phaseset. This patch fixes the issue and makes the
reference count right so that these objects can be properly garbage collected.

mercurial/parsers.c

@@ -1113,6 +1113,7 b' static PyObject *compute_phases_map_sets'
 	PyObject *phaseroots = NULL;
 	PyObject *phaseset = NULL;
 	PyObject *phasessetlist = NULL;
+	PyObject *rev = NULL;
 	Py_ssize_t len = index_length(self) - 1;
 	Py_ssize_t numphase = 0;
 	Py_ssize_t minrevallphases = 0;
@@ -1172,7 +1173,9 b' static PyObject *compute_phases_map_sets'
 		 * is computed as a difference */
 		if (phase != 0) {
 			phaseset = PyList_GET_ITEM(phasessetlist, phase);
-			PySet_Add(phaseset, PyInt_FromLong(i));
+			rev = PyInt_FromLong(i);
+			PySet_Add(phaseset, rev);
+			Py_XDECREF(rev);
 		}
 		PyList_SET_ITEM(phaseslist, i, PyInt_FromLong(phase));
 	}