Merge branch 'ps/setup-split-discovery-and-setup'

Merge branch 'ps/setup-split-discovery-and-setup'

The repository discovery and repository configuration phases, which
were previously intertwined in 'setup.c', have been split.  Repository
discovery has been updated to populate a 'struct repo_discovery'
without modifying the repository state, which is then taken by
repository configuration to initialize the repository, paving the way
for clean unification of repository configuration.

* ps/setup-split-discovery-and-setup:
  setup: mark `set_git_work_tree()` as file-local
  setup: pass worktree to `init_db()`
  setup: drop redundant configuration of `startup_info->have_repository`
  setup: make repository discovery self-contained
  setup: propagate prefix via repository discovery
  setup: drop static `cwd` variable
  setup: move prefix into repository
  setup: embed repository format in discovery
  setup: introduce explicit repository discovery
  setup: split up concerns of `setup_git_env_internal()`
  setup: unify setup of shallow file
  setup: mark bogus worktree in `apply_repository_format()`
  setup: rename `check_repository_format_gently()`