branch -d: refuse deleting a branch which is currently checked out

branch -d: refuse deleting a branch which is currently checked out

When a branch is checked out by current working tree, deleting the
branch is forbidden. However when the branch is checked out only by
other working trees, deleting incorrectly succeeds.
Use find_shared_symref() to check if the branch is in use, not just
comparing with the current working tree's HEAD.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Kazuki Yamaguchi <k@rhe.jp>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>