Call update_depot_tools and re-invoke from git-crup, to get latest
updates.

Move more of the target os functionality into git-crup.sh, so there's
no longer a need for a first-checkout script.

`git submodule sync` is horribly slow on windows.  Replicate the
functionality (but faster) in git-crup.sh.

TBR=mmoss@chromium.org,cmp@chromium.org
Review URL: https://codereview.chromium.org/11709003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@174747 0039d316-1c4b-4281-b951-d872f2087c98