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

src/applications/differential/controller/DifferentialRevisionCloseDetailsController.php

@@ -93,7 +93,7 @@ final class DifferentialRevisionCloseDetailsController
           'href' => $obj_handle->getURI(),
         ),
         $obj_handle->getName());
-      $body_why = pht(
+      $body_why[] = pht(
         'This commit and the active diff of %s had the same %s hash '.
         '(%s) so we linked this commit to %s.',
         $diff_link,

src/applications/diffusion/query/lowlevel/DiffusionLowLevelCommitFieldsQuery.php

@@ -67,19 +67,23 @@ final class DiffusionLowLevelCommitFieldsQuery
         ->withCommitHashes($hash_list)
         ->execute();
 
-      if (!empty($revisions)) {
+      if ($revisions) {
         $revision = $this->pickBestRevision($revisions);
+
         $fields['revisionID'] = $revision->getID();
         $revision_hashes = $revision->getHashes();
+
         $revision_hashes = DiffusionCommitHash::convertArrayToObjects(
           $revision_hashes);
-        $revision_hashes = mpull($revision_hashes, 'getHashType');
+        $revision_hashes = mpull($revision_hashes, null, 'getHashType');
+
         // sort the hashes in the order the mighty
         // @{class:ArcanstDifferentialRevisionHash} does; probably unnecessary
         // but should future proof things nicely.
         $revision_hashes = array_select_keys(
           $revision_hashes,
           ArcanistDifferentialRevisionHash::getTypes());
+
         foreach ($hashes as $hash) {
           $revision_hash = idx($revision_hashes, $hash->getHashType());
           if (!$revision_hash) {