Unset XDG_CONFIG_HOME and related variables during tests
Unset XDG_CONFIG_HOME and related variables during tests

If set by the user's environment, `git config --global` writes will go
to that destination instead of temporary $HOME. We definitely don't want
that.

Fixes #742

pyenv/pyenv,syndbg/goenv,jeremy/rbenv,sebhahn/pyenv,mediadoneright/rbenv,cehoffman/rbenv,viclim/rbenv,jasonkarns/rbenv,zmwangx/pyenv,sstephenson/rbenv,jrolfs/nodenv,yyyc514/rbenv,viclim/rbenv,jeremy/rbenv,cehoffman/pyenv,yyyc514/rbenv,sstephenson/rbenv,docwhat/rbenv,yyyc514/rbenv,Angeldude/rbenv,OiNutter/nodenv,OiNutter/nodenv,genome-vendor/pyenv,docwhat/rbenv,jrolfs/nodenv,mediadoneright/rbenv,cehoffman/nodenv,cehoffman/rbenv,syndbg/goenv,BanzaiMan/pyenv,jmatbastos/rbenv,jmatbastos/rbenv,sebhahn/pyenv,jasonkarns/rbenv,yyuu/pyenv,jasonkarns/rbenv,cehoffman/rbenv,mediadoneright/rbenv,bradynpoulsen/rbenv,pyenv/pyenv,nodenv/nodenv,jmatbastos/rbenv,Angeldude/rbenv,Angeldude/rbenv,nodenv/nodenv,jmatbastos/rbenv,rbenv/rbenv,genome-vendor/pyenv,syndbg/goenv,zmwangx/pyenv,bradynpoulsen/rbenv,jeremy/rbenv,pyenv/pyenv,yyuu/pyenv,cehoffman/pyenv,sebhahn/pyenv,jmatbastos/rbenv,BanzaiMan/pyenv,jmatbastos/rbenv,bradynpoulsen/rbenv,viclim/rbenv,rbenv/rbenv,cehoffman/nodenv,jmatbastos/rbenv,syndbg/goenv,docwhat/rbenv,sstephenson/rbenv