Adding unknownDocument

Author: schmidt-sebastian

Firestore/Protos/protos/firestore/local/maybe_document.proto

@@ -20,14 +20,22 @@ message NoDocument {
   google.protobuf.Timestamp read_time = 2;
 }
 
-// Represents either an existing document or the explicitly known absence of a
-// document.
+// Represents either an existing document, the explicitly known absence of a document, or a document
+// that is known to exist (at some version) but whose contents are unknown
 message MaybeDocument {
   oneof document_type {
     // Used if the document is known to not exist.
     NoDocument no_document = 1;
 
     // The document (if it exists).
     google.firestore.v1beta1.Document document = 2;
+
+    // Used if the document is known to exist but its data is unknown
+    UnknownDocument unknown_document = 3;
   }
+
+  // `hasCommittedMutations` marks documents that were written to the remote socument store based
+  // on a write acknowledgment. These documents are potentially inconsistent with the  backend's
+  // copy and use the write's commit version as their document version.
+  bool hasCommittedMutations = 4;
 }