Add shorthand parameters.

Stop changing the logging level in gcl and some logging changes.

Change SCM.GetCheckoutRoot() to SCM.checkout_root.

TEST=none
BUG=none

Review URL: http://codereview.chromium.org/523094

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@35683 0039d316-1c4b-4281-b951-d872f2087c98