Differential: exclude fields from commit message to follow our guidelines

Do it as a tweaks to render due to:

- Those fields are considered permanent.
- We still want to be able to specify them when creating new revisions
  (at least for subscribers/reviewers).

src/applications/differential/field/DifferentialAuditorsCommitMessageField.php

@@ -42,6 +42,12 @@ final class DifferentialAuditorsCommitMessageField
   }
 
   public function renderFieldValue($value) {
+    // Blender: don't include this in commit messages.
+    //
+    // NOTE: Do it at render time to match behavior of other tweaked fields
+    // (such as reviewers) which are considered non-disableable.
+    return null;
+
     return $this->renderHandleList($value);
   }
 

src/applications/differential/field/DifferentialReviewersCommitMessageField.php

@@ -62,6 +62,12 @@ final class DifferentialReviewersCommitMessageField
   }
 
   public function renderFieldValue($value) {
+    // Blender: don't include this in commit messages.
+    //
+    // NOTE: Do it at render time to allow specifying reviewers when creating
+    // a new differential revision with `arc diff`.
+    return null;
+
     $value = $this->inflateReviewers($value);
 
     $phid_list = array();

src/applications/differential/field/DifferentialSubscribersCommitMessageField.php

@@ -45,6 +45,12 @@ final class DifferentialSubscribersCommitMessageField
   }
 
   public function renderFieldValue($value) {
+    // Blender: don't include this in commit messages.
+    //
+    // NOTE: Do it at render time to allow specifying subsribers when creating
+    // a new differential revision with `arc diff`.
+    return null;
+
     return $this->renderHandleList($value);
   }
 

src/applications/differential/field/DifferentialTagsCommitMessageField.php

@@ -51,6 +51,12 @@ final class DifferentialTagsCommitMessageField
   }
 
   public function renderFieldValue($value) {
+    // Blender: don't include this in commit messages.
+    //
+    // NOTE: Do it at render time to allow specifying tags when creating
+    // a new differential revision with `arc diff`.
+    return null;
+
     return $this->renderHandleList($value);
   }