PR6712: Ignore (-traverse) common VCS directories in ocamlbuild.

From: Peter Zotov <whitequark@whitequark.org>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15668 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
update Changes

From: Gabriel Scherer <gabriel.scherer@gmail.com>

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@15669 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02