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>

git_config.py

@@ -349,9 +349,9 @@ class GitConfig(object):
     except OSError:
       return None
     try:
-      Trace(': parsing %s', self.file)
-      with open(self._json) as fd:
-        return json.load(fd)
+      with Trace(': parsing %s', self.file):
+        with open(self._json) as fd:
+          return json.load(fd)
     except (IOError, ValueError):
       platform_utils.remove(self._json, missing_ok=True)
       return None