Part of a larger refactoring to abstract SCM-specific bits.

presubmit_support, revert, gcl: modify to import gclient_scm and gclient_utils

Part of a larger refactoring to abstract SCM-specific bits.

revert, gcl: modify to import gclient_scm and gclient_utils

Part of a larger refactoring to abstract SCM-specific bits.

gclient: pull out SCM bits

Pulled out SCMWrapper into gcliet_scm.py as part of a larger refactoring to
abstract SCM-specific bits. Plan is to evenutally add git support.

Pulling out SCMWrapper also required pulling out utility functions into
a gclient_utility.py.

Patch contributed by msb@

TEST=none
BUG=none


git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@26423 0039d316-1c4b-4281-b951-d872f2087c98