When cgit learned to setup environment variables for certain repo
settings before invoking a filter process, the setup occurred inside
cgit_open_filter().
This patch moves the setup out of cgit_open_filter() and into
prepare_repo_cmd() to prepare for additional uses of these variables.
Reviewed-by: Ferry Huberts <mailings@hupie.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>