Fix diff header on renamed or copied files

Fix diff header on renamed or copied files

When git-core renames or copies a file and the mode differs the
header shows the mode change first, then the rename or copy data:

  diff --git a/COPYING b/LICENSE
  old mode 100644
  new mode 100755
  similarity index 92%
  rename from COPYING
  rename to LICENSE
  index d645695..54863be
  --- a/COPYING
  +++ b/LICENSE
  @@ -56,20 +56,6 @@

JGit relies on this ordering inside of FileHeader. Parsing "new file
mode NNN" after "copy from/to" or "rename from/to" resets the change
type to be ADD, losing the COPIED or RENAMED status and old path.

This fixes a 4 year old bug in Gerrit Code Review that prevents
opening a file for review if the file was copied from another file,
modified in this change, and the mode was updated (e.g. execute
bit was added).

Change-Id: If4c9ecd61ef0ca8e3e1ea857301f7b5c948efb96
[ms: added test case]
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>