diff-mode is able to better handle file headers

diff-mode is able to better handle file headers

This fixes a regression introduced in

  http://git.savannah.gnu.org/gitweb/?p=emacs.git;a=commit;h=2c8a7e50d24daf19ea7d86f1cfeaa98a41c56085

This bug was filed in

  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=25105

Patches generated from a VCS such as git contain a patch message at the
start, and diff-mode is now once-again able to properly able to ignore
this message when issuing navigation commands around the message.

* lisp/vc/diff-mode.el (diff-beginning-of-file-and-junk): More
  thoroughly ignore the header when looking for a beginning of file
  diffs.