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>

run_tests

@@ -20,6 +20,7 @@ import os
 import shutil
 import subprocess
 import sys
+import repo_trace
 
 
 def find_pytest():