worktree add: introduce "try --orphan" hint

Add a new advice/hint in `git worktree add` for when the user
tries to create a new worktree from a reference that doesn't exist.

Current Behavior:

% git init foo
Initialized empty Git repository in /path/to/foo/
% touch file
% git -C foo commit -q -a -m "test commit"
% git -C foo switch --orphan norefbranch
% git -C foo worktree add newbranch/
Preparing worktree (new branch 'newbranch')
fatal: invalid reference: HEAD
%

New Behavior:

% git init --bare foo
Initialized empty Git repository in /path/to/foo/
% touch file
% git -C foo commit -q -a -m "test commit"
% git -C foo switch --orphan norefbranch
% git -C foo worktree add newbranch/
Preparing worktree (new branch 'newbranch')
hint: If you meant to create a worktree containing a new orphan branch
hint: (branch with no commits) for this repository, you can do so
hint: using the --orphan option:
hint:
hint:   git worktree add --orphan newbranch/
hint:
hint: Disable this message with "git config advice.worktreeAddOrphan false"
fatal: invalid reference: HEAD
% git -C foo worktree add -b newbranch2 new_wt/
Preparing worktree (new branch 'newbranch')
hint: If you meant to create a worktree containing a new orphan branch
hint: (branch with no commits) for this repository, you can do so
hint: using the --orphan option:
hint:
hint:   git worktree add --orphan -b newbranch2 new_wt/
hint:
hint: Disable this message with "git config advice.worktreeAddOrphan false"
fatal: invalid reference: HEAD
%

Signed-off-by: Jacob Abel <jacobabel@nullpo.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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