When the git default branch is not set to master the installcheck test suite fails. This patch adjusts the test setup scripts to ignore the system and user git config files. GIT_CONFIG_SYSTEM is set to /dev/null to ignore /etc/gitconfig GIT_CONFIG_GLOBAL is not set because the global config files are loaded from $HOME or $XDG_CONFIG_HOME which we already reset. git documentation: https://git-scm.com/docs/git#Documentation/git.txt-codeGITCONFIGGLOBALcode Change-Id: Ie73bbed1db9419c9885b9d57e4edb7a4047d5cce