git switch error message tweak

To help those migrating from 'git checkout' to the newer 'git switch'
and 'git restore', this error message could be improved:

% git switch baa022666d26a5d40e86c0b6349eb7435033e80a
fatal: a branch is expected, got commit 'baa022666d26a5d40e86c0b6349eb7435033e80a'

It could print additionally

To switch to this commit in 'detached HEAD' mode, use --detach.

as long as it was indeed a commit that was given.

I note also that while git checkout prints a spiel about "You are in
'detached HEAD' state", git switch --detach does not.  Was this
deliberate, on the assumption that if you pass --detach you know what
you are doing?  I think it might be better to honour the
advice.detachedHead setting the same way for both checkout and switch.

Thanks for considering this request, and apologies if this is something already discussed and decided.

-- 
Ed Avis [off-list ref]