tests: test_install commit and clean

Uncommited files wouldn't be accessible as git objects.

It needs `git clean -fd` to amend "Working Tree" metrics collection
which took its part when the git tree is dirty.

tests/test_install.py

@@ -20,6 +20,8 @@ class TestInstall(TestDvc):
         self.assertTrue(os.path.isfile(hook("pre-commit")))
 
         self.dvc.add(self.FOO)
+        self.dvc.scm.add([".gitignore", self.FOO + ".dvc"])
+        self.dvc.scm.commit("add")
         os.unlink(self.FOO)
 
         self.dvc.scm.checkout("branch", create_new=True)

tests/test_metrics.py

@@ -489,7 +489,7 @@ class TestCachedMetrics(TestDvc):
         self.assertIsNotNone(stage)
         self.assertEqual(stage.relpath, "metrics.json.dvc")
 
-        self.dvc.scm.add([".gitignore", "metrics.json.dvc"])
+        self.dvc.scm.add(["code.py", ".gitignore", "metrics.json.dvc"])
         self.dvc.scm.commit(branch)
 
     def _test_metrics(self, func):
@@ -502,6 +502,10 @@ class TestCachedMetrics(TestDvc):
         func("one")
         func("two")
 
+        # TestDvc currently is based on TestGit, so it is safe to use
+        # scm.git for now
+        self.dvc.scm.git.git.clean("-fd")
+
         self.dvc = DvcRepo(".")
 
         res = self.dvc.metrics.show(