Fix gclient diff & pack to not throw uncatched exceptions when a directory is missing.

Reorder things in gclient.py for easier diff later
Update revinfo help
Add GetScmName(), it will be needed later.

TEST=unit tests
BUG=none

Review URL: http://codereview.chromium.org/2786013

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@49565 0039d316-1c4b-4281-b951-d872f2087c98