This solves a problem with "os.rename" sometimes failing with an
exception after cloning a dependency to a temporary directory. It's
possible the dying git processes still hold a handle to the directory.
Since gclient delete the temporary directory regardless of the success
of the process, it results in a lot of GB downloaded for nothing.

SafeRename solves this by retrying a few times if the renaming fails,
sleeping one second every time to get other processes the time to
release their lock on the directory. It gives up retrying after 15 times.

BUG=
R=maruel@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23875041

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@224372 0039d316-1c4b-4281-b951-d872f2087c98