Squash default

The commit messages explain this better, but basically:

1. Configure sparse checkout for the worktrees we use for squashing (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1024#note_22844112 / https://gitlab.com/gitlab-org/gitlab-ee/issues/1726#note_23874661).
2. Improve the logging of errors to make them easier to find for a particular MR.
3. Change the default commit message and author to be the MR title and author, until https://gitlab.com/gitlab-org/gitlab-ee/issues/1510 is done.
4. Skip git hooks when squashing, to avoid the pre-commit hook added by git annex: https://gitlab.com/gitlab-org/gitlab-ee/issues/1726#note_23874661