Merge branch 'rj/avoid-switching-to-already-used-branch'

A few subcommands have been taught to stop users from working on a
branch that is being used in another worktree linked to the same
repository.

* rj/avoid-switching-to-already-used-branch:
  switch: reject if the branch is already checked out elsewhere (test)
  rebase: refuse to switch to a branch already checked out elsewhere (test)
  branch: fix die_if_checked_out() when ignore_current_worktree
  worktree: introduce is_shared_symref()

diff --cc branch.c
Simple merge

diff --cc worktree.c
Simple merge

diff --cc worktree.h
Simple merge