This new method allows us to install pre-cl-upload
and pre-cl-dcommit hook files which should rarely need
to be updated (with luck).  These hooks load the real
hooks lib from depot_tools which takes over execution
of the presubmit tests.

BUG=5339
TEST=hooks are run by git cl upload/dcommit and behave
as expected
Review URL: http://codereview.chromium.org/243118

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@28409 0039d316-1c4b-4281-b951-d872f2087c98