Fix last_n keep rule (#691) (#750)

From https://github.com/zrepl/zrepl/issues/691

The last_n prune rule keeps everything, regardless of if it matches the
regex or not, if there are less than count snapshot. The expectation
would be to never keep non-regex snapshots, regardless of number.

pruning/keep_last_n_test.go

@@ -90,7 +90,7 @@ func TestKeepLastN(t *testing.T) {
 				stubSnap{"a2", false, o(12)},
 			},
 			rules: []KeepRule{
-				MustKeepLastN(3, "a"),
+				MustKeepLastN(4, "a"),
 			},
 			expDestroy: map[string]bool{
 				"b1": true,