Prefix error events with RepoErrorEvent:

Prior to this change there is no way to distinguish between git sessions logs
generated from repo source v.s. from git.

Bug: b/294446468
Change-Id: I309f59e146c30cb08a0637e8d0b9c5d9efd5cada
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/381794
Commit-Queue: Jason Chang <jasonnc@google.com>
Reviewed-by: Joanna Wang <jojwang@google.com>
Tested-by: Jason Chang <jasonnc@google.com>

git_trace2_event_log.py

@@ -203,8 +203,8 @@ class EventLog(object):
         error_event = self._CreateEventDict("error")
         if fmt is None:
             fmt = msg
-        error_event["msg"] = msg
-        error_event["fmt"] = fmt
+        error_event["msg"] = f"RepoErrorEvent:{msg}"
+        error_event["fmt"] = f"RepoErrorEvent:{fmt}"
         self._log.append(error_event)
 
     def _GetEventTargetPath(self):