worktree add: extend DWIM to infer --orphan

Extend DWIM to try to infer `--orphan` when in an empty repository. i.e.
a repository with an invalid/unborn HEAD, no local branches, and if
`--guess-remote` is used then no remote branches.

This behavior is equivalent to `git switch -c` or `git checkout -b` in
an empty repository.

Also warn the user (overriden with `-f`/`--force`) when they likely
intend to checkout a remote branch to the worktree but have not yet
fetched from the remote. i.e. when using `--guess-remote` and there is a
remote but no local or remote refs.

Current Behavior:
% git --no-pager branch --list --remotes
% git remote
origin
% git workree add ../main
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
% git workree add --guess-remote ../main
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
% git fetch --quiet
% git --no-pager branch --list --remotes
origin/HEAD -> origin/main
origin/main
% git workree add --guess-remote ../main
Preparing worktree (new branch 'main')
branch 'main' set up to track 'origin/main'.
HEAD is now at dadc8e6dac commit message
%

New Behavior:
% git --no-pager branch --list --remotes
% git remote
origin
% git workree add ../main
No possible source branch, inferring '--orphan'
Preparing worktree (new branch 'main')
% git worktree remove ../main
% git workree add --guess-remote ../main
fatal: No local or remote refs exist despite at least one remote
present, stopping; use 'add -f' to overide or fetch a remote first
% git workree add --guess-remote -f ../main
No possible source branch, inferring '--orphan'
Preparing worktree (new branch 'main')
% git worktree remove ../main
% git fetch --quiet
% git --no-pager branch --list --remotes
origin/HEAD -> origin/main
origin/main
% git workree add --guess-remote ../main
Preparing worktree (new branch 'main')
branch 'main' set up to track 'origin/main'.
HEAD is now at dadc8e6dac commit message
%

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

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