rename local trace module

There is a standard Python "trace" module, so having a local trace.py
prevents us being able to import that.  Rename the module to avoid.

Change-Id: I23e29ec95a2204bb168a641323d05e76968d9b57
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/234832
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

repo_trace.py

@@ -0,0 +1,40 @@
+# -*- coding:utf-8 -*-
+#
+# Copyright (C) 2008 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Logic for tracing repo interactions.
+
+Activated via `repo --trace ...` or `REPO_TRACE=1 repo ...`.
+"""
+
+from __future__ import print_function
+import sys
+import os
+
+# Env var to implicitly turn on tracing.
+REPO_TRACE = 'REPO_TRACE'
+
+_TRACE = os.environ.get(REPO_TRACE) == '1'
+
+def IsTrace():
+  return _TRACE
+
+def SetTrace():
+  global _TRACE
+  _TRACE = True
+
+def Trace(fmt, *args):
+  if IsTrace():
+    print(fmt % args, file=sys.stderr)