ship a default gitignore
Currently git shows package files, signatures and logs as untracked files and would happily commit them. We now ignore these by default.
This requires bumping the repo spec, so we also use that chance to add empty (global) git hooks, which we can fill in the future without the need of re-configuring existing repos.
BREAKING CHANGE: This change modifies GIT_REPO_SPEC_VERSION, so one needs to run 'pkgctl repo configure' on previously cloned repos.
Fixes #129 (closed)