depot_tools' git wrapper for some reason overrides %HOME% to point to
depot_tools directory, and thus scripts running as git subprocesses (git_cl.py)
use not the same HOME as scripts that run outside of git process tree
(depot-tools-auth.py). Instead always use USERPROFILE in auth.py to consistently
pick same directory.

This issue happens only when HOME is not set globally. It can explain why it
wasn't seen earlier.

R=iannucci@chromium.org
BUG=356813

Review URL: https://codereview.chromium.org/1082833004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@294911 0039d316-1c4b-4281-b951-d872f2087c98