We're seeing fetches fail in interesting ways:
running "git fetch -v --progress origin +refs/heads/*:refs/heads/*" in "/mnt/scratch0/b_used/build/slave/cache_dir/chrome--internal.googlesource.com-chrome-src--internal"
error: object file ./objects/a1/4bd89aa4cc7d7bbad7594cba0ae73e99dbb54c is empty
error: object file ./objects/a1/4bd89aa4cc7d7bbad7594cba0ae73e99dbb54c is empty
fatal: loose object a14bd89aa4cc7d7bbad7594cba0ae73e99dbb54c (stored in ./objects/a1/4bd89aa4cc7d7bbad7594cba0ae73e99dbb54c) is corrupt
fatal: The remote end hung up unexpectedly

And then the cache becomes corrupted.  This blows the cache away if this happens.

BUG=261741

Review URL: https://codereview.chromium.org/352543003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@280123 0039d316-1c4b-4281-b951-d872f2087c98