The root of problem is a _cache_temp file.
git_cache expected that it is a folder.
So rmtree failed to remove it.

BUG=
TBR= dpranke@chromium.org, enne@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/825133002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@293502 0039d316-1c4b-4281-b951-d872f2087c98