Add an option in DEPS files to clone a project into a temp dir
and then copy into expected final dir.  This allows checking out a
git repo into a folder which is non-empty. It is useful for projects
that are embedded in src/ but want to specify the revision of
src/ in the embedded project (such as android private).

BUG=165280

Review URL: https://chromiumcodereview.appspot.com/19359002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@212720 0039d316-1c4b-4281-b951-d872f2087c98