Update draft release notes to 1.7.8.4

Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/RelNotes/1.7.8.4.txt

@@ -8,6 +8,11 @@ Fixes since v1.7.8.3
    directory when two paths in question are in adjacent directories and
    the name of the one directory is a prefix of the other.
 
+ * When producing a "thin pack" (primarily used in bundles and smart
+   HTTP transfers) out of a fully packed repository, we unnecessarily
+   avoided sending recent objects as a delta against objects we know
+   the other side has.
+
  * "git send-email" did not properly treat sendemail.multiedit as a
    boolean (e.g. setting it to "false" did not turn it off).