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>

git_command.py

@@ -23,7 +23,7 @@ from signal import SIGTERM
 
 from error import GitError
 import platform_utils
-from trace import REPO_TRACE, IsTrace, Trace
+from repo_trace import REPO_TRACE, IsTrace, Trace
 from wrapper import Wrapper
 
 GIT = 'git'

git_config.py

@@ -44,7 +44,7 @@ else:
 from signal import SIGTERM
 from error import GitError, UploadError
 import platform_utils
-from trace import Trace
+from repo_trace import Trace
 if is_python3():
   from http.client import HTTPException
 else:

git_refs.py

@@ -15,7 +15,7 @@
 # limitations under the License.
 
 import os
-from trace import Trace
+from repo_trace import Trace
 import platform_utils
 
 HEAD      = 'HEAD'

main.py

@@ -45,7 +45,7 @@ except ImportError:
 
 from color import SetDefaultColoring
 import event_log
-from trace import SetTrace
+from repo_trace import SetTrace
 from git_command import git, GitCommand
 from git_config import init_ssh, close_ssh
 from command import InteractiveCommand
@@ -84,7 +84,7 @@ global_options.add_option('--color',
                           help='control color usage: auto, always, never')
 global_options.add_option('--trace',
                           dest='trace', action='store_true',
-                          help='trace git command execution')
+                          help='trace git command execution (REPO_TRACE=1)')
 global_options.add_option('--time',
                           dest='time', action='store_true',
                           help='time repo command execution')

progress.py

@@ -17,7 +17,7 @@
 import os
 import sys
 from time import time
-from trace import IsTrace
+from repo_trace import IsTrace
 
 _NOT_TTY = not os.isatty(2)
 

project.py

@@ -39,7 +39,7 @@ from error import GitError, HookError, UploadError, DownloadError
 from error import ManifestInvalidRevisionError
 from error import NoManifestException
 import platform_utils
-from trace import IsTrace, Trace
+from repo_trace import IsTrace, Trace
 
 from git_refs import GitRefs, HEAD, R_HEADS, R_TAGS, R_PUB, R_M
 

repo_trace.py

@@ -14,15 +14,19 @@
 # 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'
 
-try:
+_TRACE = os.environ.get(REPO_TRACE) == '1'
-  _TRACE = os.environ[REPO_TRACE] == '1'
-except KeyError:
-  _TRACE = False
 
 def IsTrace():
   return _TRACE