cram: Avoid globally configured git filters tripping up tests
cram: Avoid globally configured git filters tripping up tests

build.t makes sure no extra files are created in the cache directory.
If global git config sets a filter, said filter is triggered by the
code fetched by "go get", and the filter is implemented by a demand
script, the resulting executable will end up there, too.

Overriding XDG_CONFIG_HOME makes git not use the global configuration,
and avoids this issue. Hopefully the "go get" completes without git
configuration.

tv42/demand,tv42/demand