worktree add: emit warn when there is a bad HEAD

worktree add: emit warn when there is a bad HEAD

Add a warning to `worktree add` when the command tries to reference
HEAD, there exist valid local branches, and the HEAD points to a
non-existent reference.

Current Behavior:
% git -C foo worktree list
/path/to/repo/foo     dadc8e6dac [main]
/path/to/repo/foo_wt  0000000000 [badref]
% git -C foo worktree add ../wt1
Preparing worktree (new branch 'wt1')
HEAD is now at dadc8e6dac dummy commit
% git -C foo_wt worktree add ../wt2
hint: If you meant to create a worktree containing a new orphan branch
[...]
hint: Disable this message with "git config advice.worktreeAddOrphan false"
fatal: invalid reference: HEAD
%

New Behavior:
% git -C foo worktree list
/path/to/repo/foo     dadc8e6dac [main]
/path/to/repo/foo_wt  0000000000 [badref]
% git -C foo worktree add ../wt1
Preparing worktree (new branch 'wt1')
HEAD is now at dadc8e6dac dummy commit
% git -C foo_wt worktree add ../wt2
warning: HEAD points to an invalid (or orphaned) reference.
HEAD path: '/path/to/repo/foo/.git/worktrees/foo_wt/HEAD'
HEAD contents: 'ref: refs/heads/badref'
hint: If you meant to create a worktree containing a new orphan branch
[...]
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>