Create an update_depot_tools.bat file to use for updated when on cygwin, and
properly call it from the update_depot_tools shell script and gclient batch
files.

BUG=none
TEST=gclient sync and git cl work again on cygwin.  Can also turn echo on in update_depot_tools.bat to verify updating working correctly.

Review URL: http://codereview.chromium.org/3856006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@63234 0039d316-1c4b-4281-b951-d872f2087c98