[geary/wip/conversation-polish] Include preview in Geary.Email.MUTABLE_FIELDS

[Michael Gratton <mjog src gnome org>]

commit 049abb4b9fe5a40c6f95902af74a4be3f6df2233
Author: Michael Gratton <mike vee net>
Date:   Fri Feb 1 11:40:51 2019 +1100

    Include preview in Geary.Email.MUTABLE_FIELDS
    
    This means that once the full message body has been retreived, the
    possibly partial or poor preview will be updated on merge into the DB.
    
    See Bug 714317 for historical context.

 src/engine/api/geary-email.vala | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)
---
diff --git a/src/engine/api/geary-email.vala b/src/engine/api/geary-email.vala
index 5f45af4d..1c62e9bf 100644
--- a/src/engine/api/geary-email.vala
+++ b/src/engine/api/geary-email.vala
@@ -36,10 +36,15 @@ public class Geary.Email : BaseObject {
     /**
      * Indicates email fields that may change over time.
      *
-     * Currently only one field is mutable: FLAGS. All others never
-     * change once stored in the database.
+     * The mutable fields are: FLAGS -- since these change as for
+     * example messages are marked as read, and PREVIEW -- since the
+     * preview is updated when the full message body is
+     * available. All others never change once stored in the
+     * database.
      */
-    public const Field MUTABLE_FIELDS = Geary.Email.Field.FLAGS;
+    public const Field MUTABLE_FIELDS = (
+        Geary.Email.Field.FLAGS | Geary.Email.Field.PREVIEW
+    );
 
     /**
      * Indicates the email fields required to build an RFC822.Message.