Bare-repository worktrees resolve their git common dir to the bare repo
itself (for example /repos/foo.git), not to a directory literally named
.git. The first fix only collapsed non-bare linked worktrees, so bare
worktree layouts still fell back to each individual worktree root.
Return the shared commonDir for bare repositories from both
repo.primaryRoot and repo.primaryRootSync, update project-label docs,
and cover two worktrees attached to one bare repository in the
hindsight bank regression test.
Fixes#2232
