worktree.c: make find_shared_symref() return struct worktree *

This gives the caller more information and they can answer things like,
"is it the main worktree" or "is it the current worktree". The latter
question is needed for the "checkout a rebase branch" case later.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>