Introduce git__git_dir() helper
Unset GIT_DIR
Bugs propagation is now done by post-rewrite git hook

There's now no need to overload all git commands that rewritte commits.

The downside is that a manual step is needed to switch to this new
infrastructure.

Since hooks are installed during project installation, you can use
kdist-project--update in an existing project.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>