This prevents jgit from parsing the user's existing ~/.gitconfig,
which can skew test results.

BUG=

Review URL: https://codereview.chromium.org/27752003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@229245 0039d316-1c4b-4281-b951-d872f2087c98