remotefilelog: fix ctreemanifest serialization

Summary:
The serialization was completely broken because std::string.append("\0") treated
the argument as a c-string which was completely empty. Let's use the
append(char*, size_t) version instead to make sure it appends the correct
characters.

Also makes othere uses of append in that function match the same pattern.

Test Plan: Later patches worked

Reviewers: #fastmanifest

Subscribers: mjpieters

Differential Revision: https://phabricator.intern.facebook.com/D3885014

remotefilelog/ctreemanifest/manifest.cpp

@@ -202,12 +202,12 @@ void Manifest::serialize(std::string &result) {
   ManifestEntry *entry;
   while ((entry = iterator.next()) != NULL) {
     result.append(entry->filename, entry->filenamelen);
-    result.append("\0");
+    result.append("\0", 1);
     result.append(entry->node, HEX_NODE_SIZE);
     if (entry->flag) {
-      result.append(1, *entry->flag);
+      result.append(entry->flag, 1);
     }
-    result.append("\n");
+    result.append("\n", 1);
   }
 }