Summary:
Fixes T5534. If you `git push origin :refs/tags/doesnotexist` (for some non-existing tag), we get a change where both the old and new refs are empty.
We incorrectly call this an "add", because the old ref is empty. Instead, call this a "delete", but skip the logic which would normally mark it dangerous.
(Possibly we should just reject these outright, but Git allows them, so stick with that for now.)
Test Plan:
Pushed nonexistent refs:
```
$ git push origin :refs/tags/doesnotexist
remote: warning: Allowing deletion of corrupt ref.
To ssh://dweller@localhost/diffusion/POEMS/
- [deleted] doesnotexist
$
```
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T5534
Differential Revision: https://secure.phabricator.com/D9800
02c3200867