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>