git-map: Show your local repo's history in a pseudo-graphical format from the command line.

git-map-branches: Show the topology of all of your branches, and their upstream relationships.

git-nav-upstream: Navigate (checkout) to the upstream branch of the current branch.

git-nav-downstream: Navigate (checkout) to a downstream branch of the current branch. If there's more than one downstream branch, then present a menu to select which one you want.

R=agable@chromium.org, hinoka@chromium.org, stip@chromium.org, szager@chromium.org
BUG=

Review URL: https://codereview.chromium.org/184113002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@256384 0039d316-1c4b-4281-b951-d872f2087c98