Add repo_checkout to recipe_util.
Add repo_checkout to recipe_util.

This provides simple functionality for projects that use repo to manage their
git repositories. Tests forthcoming in a patch soon.

R=iannucci@chromium.org

Review URL: https://chromiumcodereview.appspot.com/16253004

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@203510 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build