build: use absolute dir for GIT_DIR/GIT_WORK_TREE
build: use absolute dir for GIT_DIR/GIT_WORK_TREE

Setting GIT_WORK_TREE to a relative directory resulted in
"error: packfile ... cannot be accessed" error. Either GIT_WORK_TREE
should be relative to GIT_DIR, i.e. ../, or must be an absolute dir

kierank/bmxlib-bmx,kierank/bmxlib-bmx,stuarthicks/bmx,Limecraft/ebu-bmx,stuarthicks/bmx,Limecraft/ebu-bmx,kierank/bmxlib-bmx,xunchangguo/BMX,Limecraft/ebu-bmx,stuarthicks/bmx,xunchangguo/BMX,xunchangguo/BMX