With this change, 'git cache fetch' will automatically re-bootstrap
a repo from google storage if the local cache has too many pack
files.  The behavior can be disabled with the --no-bootstrap flag.

BUG=

Review URL: https://codereview.chromium.org/1320383004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296829 0039d316-1c4b-4281-b951-d872f2087c98