Set tracing to always on and save to .repo/TRACE_FILE.

- add `--trace_to_stderr` option so stderr will include trace outputs and any other errors that get sent to stderr
- while TRACE_FILE will only include trace outputs

piggy-backing on: https://gerrit-review.googlesource.com/c/git-repo/+/349154

Change-Id: I3895a84de4b2784f17fac4325521cd5e72e645e2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/350114
Reviewed-by: LaMont Jones <lamontjones@google.com>
Tested-by: Joanna Wang <jojwang@google.com>

tests/test_project.py

@@ -26,6 +26,7 @@ import git_command
 import git_config
 import platform_utils
 import project
+import repo_trace
 
 
 @contextlib.contextmanager
@@ -64,6 +65,13 @@ class FakeProject(object):
 class ReviewableBranchTests(unittest.TestCase):
   """Check ReviewableBranch behavior."""
 
+  def setUp(self):
+    self.tempdirobj = tempfile.TemporaryDirectory(prefix='repo_tests')
+    repo_trace._TRACE_FILE = os.path.join(self.tempdirobj.name, 'TRACE_FILE_from_test')
+
+  def tearDown(self):
+    self.tempdirobj.cleanup()
+
   def test_smoke(self):
     """A quick run through everything."""
     with TempGitTree() as tempdir: