I tried a git-ish tree/blob model, but way too slow for a ton of files.
This maps easily on to the current way the tree is mapped, saves
temp space on bots (because the full isos aren't downloaded), and is
a lot faster too. It means we can pull old revisions too now.

TBR=iannucci@chromium.org
BUG=348350

Review URL: https://codereview.chromium.org/185423004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@254421 0039d316-1c4b-4281-b951-d872f2087c98