Configure HOME for the gerrit process.
Configure HOME for the gerrit process.

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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@229245 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools