Merge branch 'ds/branch-checked-out'

Merge branch 'ds/branch-checked-out'

Introduce a helper to see if a branch is already being worked on
(hence should not be newly checked out in a working tree), which
performs much better than the existing find_shared_symref() to
replace many uses of the latter.

* ds/branch-checked-out:
  branch: drop unused worktrees variable
  fetch: stop passing around unused worktrees variable
  branch: fix branch_checked_out() leaks
  branch: use branch_checked_out() when deleting refs
  fetch: use new branch_checked_out() and add tests
  branch: check for bisects and rebases
  branch: add branch_checked_out() helper