Prefer installed leaves in runtime cycle topological sort

In order to avoid possibly merging a package too early, prefer
installed leaves in runtime cycle topological sort. This fixes
an AlternativesGzipTestCase failure that arose after 2e298ea
caused leaves to be selected in a slightly different order.

Bug: https://bugs.gentoo.org/917259
Signed-off-by: Zac Medico <zmedico@gentoo.org>

lib/_emerge/depgraph.py

@@ -9498,6 +9498,12 @@ def find_smallest_cycle(mergeable_nodes, local_priority_range):
                     if smallest_leaves is None:
                         smallest_leaves = [cycle_digraph.order[-1]]
 
+                    # Prefer installed leaves, in order to avoid
+                    # merging something too early.
+                    installed_leaves = [pkg for pkg in smallest_leaves if pkg.installed]
+                    if installed_leaves:
+                        smallest_leaves = installed_leaves
+
                     # Only harvest one node at a time, in order to
                     # minimize the number of ignored dependencies.
                     cycle_digraph.remove(smallest_leaves[0])