This is a stopgap to prevent people with >20 local branches from running tools
like 'git rebase-update'. This usually indicates that the user isn't in the
habit of cleaning up old branches, which makes these tools unlikely to be useful
in the current state of their repo anyway.

R=agable@chromium.org
BUG=

Review URL: https://codereview.chromium.org/238213006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@263978 0039d316-1c4b-4281-b951-d872f2087c98