Fix the git-* wrapper scripts, PRESUBMIT.py and .gitignore.

The previous python wrapping scripts assumed python_bin\python.exe. This CL
modifies the wrapping scripts to use python.bat. This requires some quoting
magic and explicitly defer to cmd.exe.

This may break some cases where double quotes are used as arguments. We can't do
miracles on Windows.

R=iannucci@chromium.org
BUG=

Review URL: https://codereview.chromium.org/50563008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@231888 0039d316-1c4b-4281-b951-d872f2087c98