Added the 'git-retry' bootstrap command. This can be used to wrap other 'git'
commands around a fault-tolerant retry wrapper.

BUG=295109
TEST=localtest
R=iannucci@chromium.org, petermayo@chromium.org

Review URL: https://codereview.chromium.org/401673003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@285939 0039d316-1c4b-4281-b951-d872f2087c98