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>
diff --git a/main.py b/main.py
index 5c546e8..35023d5 100755
--- a/main.py
+++ b/main.py
@@ -45,7 +45,7 @@
 
 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 @@
                           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')