With the function set_git_dir() you can reset the path that will
be used for git_path(), git_dir() and friends.
The responsibility to close files and throw away information from the
old git_dir lies with the caller.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>