The git cache command is a central place to manage a machine's git cache.

It provides two subcommands:
* populate -- creates or updates the cache of a given repository
* exists -- prints the path to the cache of a repo, if it exists

Gclient, deps2git, bot_update, and any other tools that touch the cache will
be able to use this command to make sure that everyone is interacting with
the cache in the same way.

R=hinoka@chromium.org, iannucci@chromium.org
BUG=339168

Review URL: https://codereview.chromium.org/164823002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@253007 0039d316-1c4b-4281-b951-d872f2087c98