branch: use branch_checked_out() when deleting refs

branch: use branch_checked_out() when deleting refs

This is the last current use of find_shared_symref() that can easily be
replaced by branch_checked_out(). The benefit of this switch is that the
code is a bit simpler, but also it is faster on repeated calls.

The remaining uses of find_shared_symref() are non-trivial to remove, so
we probably should not continue in that direction:

* builtin/notes.c uses find_shared_symref() with "NOTES_MERGE_REF"
  instead of "HEAD", so it doesn't have an immediate analogue with
  branch_checked_out(). Perhaps we should consider extending it to
  include that symref in addition to HEAD, BISECT_HEAD, and
  REBASE_HEAD.

* receive-pack.c checks to see if a worktree has a checkout for the ref
  that is being updated. The tricky part is that it can actually decide
  to update the worktree directly instead of just skipping the update.
  This all depends on the receive.denyCurrentBranch config option. The
  implementation currenty cares about receiving the worktree in the
  result, so the current branch_checked_out() prototype is insufficient
  currently. This is something to investigate later, though, since a
  large number of refs could be updated at the same time and using the
  strmap implementation of branch_checked_out() could be beneficial.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>