Changed behavior if --lf and --ff are both used.

When using both --last-failed/--lf and --failed-first/--ff pytest would
run all tests with failed tests first (as if --lf was not provied). This
patch changes it so that when using both flags, only the last failed
tests are run. This makes it easier to set --ff as the default behavior
via the config file and then selectively use --lf to only run the last
failed tests.

_pytest/cacheprovider.py

@@ -139,11 +139,11 @@ class LFPlugin(object):
                 # running a subset of all tests with recorded failures outside
                 # of the set of tests currently executing
                 pass
-            elif self.config.getvalue("failedfirst"):
-                items[:] = previously_failed + previously_passed
-            else:
+            elif self.config.getvalue("lf"):
                 items[:] = previously_failed
                 config.hook.pytest_deselected(items=previously_passed)
+            else:
+                items[:] = previously_failed + previously_passed
 
     def pytest_sessionfinish(self, session):
         config = self.config

testing/test_cache.py

@@ -192,13 +192,34 @@ class TestLastFailed(object):
             "test_a.py*",
             "test_b.py*",
         ])
-        result = testdir.runpytest("--lf", "--ff")
+        result = testdir.runpytest("--ff")
         # Test order will be failing tests firs
         result.stdout.fnmatch_lines([
             "test_b.py*",
             "test_a.py*",
         ])
 
+    def test_lastfailed_failedfirst_order(self, testdir):
+        testdir.tmpdir.join('test_a.py').write(_pytest._code.Source("""
+            def test_always_passes():
+                assert 1
+        """))
+        testdir.tmpdir.join('test_b.py').write(_pytest._code.Source("""
+            def test_always_fails():
+                assert 0
+        """))
+        result = testdir.runpytest()
+        # Test order will be collection order; alphabetical
+        result.stdout.fnmatch_lines([
+            "test_a.py*",
+            "test_b.py*",
+        ])
+        result = testdir.runpytest("--lf", "--ff")
+        # Test order will be failing tests firs
+        result.stdout.fnmatch_lines([
+            "test_b.py*",
+        ])
+
     def test_lastfailed_difference_invocations(self, testdir, monkeypatch):
         monkeypatch.setenv("PYTHONDONTWRITEBYTECODE", 1)
         testdir.makepyfile(test_a="""