git: make sure Git runs with a consistent and reproducible config

git: make sure Git runs with a consistent and reproducible config

Arrange for GIT_CONFIG_NOSYSTEM=1 and GIT_ATTR_NOSYSTEM=1 to always
be set when running Git.

Addtionally cause XDG_CONFIG_HOME to be set to
$Config::chroot.'/var/empty' and HOME to be set to
$Config::chroot.'/etc/girocco' when running Git so that a consistent
set of config values will be seen.

After the $chroot database is set up (by running `make install`),
a $chroot/etc/girocco/.gitconfig file may be created (along with
its girocco parent directory) in order to set non-default values
for any desired config items when Girocco runs Git.