Did this sort of thing in a repository:
```
$ git checkout -b featurex
$ echo x >> y
$ git commit -am wip
$ arc diff
```
Then I simulated just pushing it (this flow is a little more involved than necessary):
```
$ arc land --hold
$ git commit --amend
$ # remove all metadata -- particularly, "Differential Revision"!
$ git push HEAD:master
```
I got a not-great but more-useful dialog:
{F1667318}
Prior to this change, the hash match was incorrectly not reported at all.