Merge branch 'stable-4.9'

* stable-4.9:
  PackInserter: Ensure objects are written at the end of the pack
  ObjectInserter: Add warning about mixing read-back with writes

Change-Id: I308e7c1c6b72e8d4d9b5d0f4f51e9815fc92d7d7

diff --cc org.eclipse.jgit.test/tst/org/eclipse/jgit/internal/storage/file/PackInserterTest.java
Simple merge

diff --cc org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/PackInserter.java
Simple merge

diff --cc org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectInserter.java
Simple merge