The "branch-heads" are refspecs created from upstream svn branches. This flag
is mostly needed for official builders, and adds about 1/2GB of data to the
checkout, so not currently enabling by default.
Review URL: https://codereview.chromium.org/12546017

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@186598 0039d316-1c4b-4281-b951-d872f2087c98