Refresh .gitignore

After svn:ignore changes, from any git-svn repository you can run

  git svn show-ignore > .gitignore

to synchronize the .gitignore content.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15087 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02