Fix some issues with the "Explain Why" dialog

Summary:
Ref T11051. This is still not as clear as it should be, but is at least working as intended now.

I believe this part of the code just never worked. The test plan on D10489 didn't specifically cover it.

Test Plan:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11051

Differential Revision: https://secure.phabricator.com/D15989