Update "evt" field to be logged as a string type.

Testing:
- Unit tests
- Verified git trace log has "evt": "2" (vs "evt": 2 previously)

Bug: https://crbug.com/gerrit/13966
Change-Id: I2e0c98dda0cccdd5cb6328105c11b93cd42676eb
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/294123
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Ian Kasprzak <iankaz@google.com>

git_trace2_event_log.py

@@ -92,7 +92,7 @@ class EventLog(object):
   def _AddVersionEvent(self):
     """Adds a 'version' event at the beginning of current log."""
     version_event = self._CreateEventDict('version')
-    version_event['evt'] = 2
+    version_event['evt'] = "2"
     version_event['exe'] = RepoSourceVersion()
     self._log.insert(0, version_event)