Lockless operation for cleaner results

Instead of sharing the .git dir with an explicit GIT_DIR/GIT_WORK_TREE,
create proper repositories, but symlink the objects dir etc. properly.
This way we don't ened locks, repositories are cleaner, and there's less
chance of buildsystems getting confused.

Fixes issues #5, #6 and #7
devserver: don't listen on localhost only
Better Debian branch detection again
Easier killing of golem master and workers