worktree.c: check whether branch is rebased in another worktree

worktree.c: check whether branch is rebased in another worktree

This function find_shared_symref() is used in a couple places:

1) in builtin/branch.c: it's used to detect if a branch is checked out
   elsewhere and refuse to delete the branch.

2) in builtin/notes.c: it's used to detect if a note is being merged in
   another worktree

3) in branch.c, the function die_if_checked_out() is actually used by
   "git checkout" and "git worktree add" to see if a branch is already
   checked out elsewhere and refuse the operation.

In cases 1 and 3, if a rebase is ongoing, "HEAD" will be in detached
mode, find_shared_symref() fails to detect it and declares "no branch is
checked out here", which is not really what we want.

This patch tightens the test. If the given symref is "HEAD", we try to
detect if rebase is ongoing. If so return the branch being rebased. This
makes checkout and branch delete operations safer because you can't
checkout a branch being rebased in another place, or delete it.

Special case for checkout. If the current branch is being rebased,
git-rebase.sh may use "git checkout" to abort and return back to the
original branch. The updated test in find_shared_symref() will prevent
that and "git rebase --abort" will fail as a result.
find_shared_symref() and die_if_checked_out() have to learn a new
option ignore_current_worktree to loosen the test a bit.

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