git worktrees must exist even if locked

["taylor, david" <David.Taylor@dell.com>]

The Git documentation in describing worktrees says that one reason
why you might want to lock a worktree is to prevent it from being pruned
if it is on a removable media that isn't currently mounted.

So, my expectation was that if the worktree is inaccessible (and locked), Git
would pretend that there is no worktree by that name.

In reality, if you have such a worktree, Git gets an error.

 On local systems, /home is local to a machine; home directories are elsewhere.
Home directories are NFS mounted; /home is not.

. create a repository in /my/home/dir/my-repo.git with

    git clone --bare <some arguments>

. create an empty directory /home/somedir/worktree-tests

. use 'git worktree add' to add /home/somedir/worktree-tests/<branch-name>
  as a worktree on branch <branch-name>.  It gets populated with the correct
  content.

. lock it using'git worktree lock'

So far, so good.  Now, go to a different computer -- one on which
/home/somedir/worktree-tests does not exist (and therefore
/home/somedir/worktree-tests/<branch-name> does not exist).

. cd /my/home/dir/my-repo.git

Now, try issuing Git commands.  Many will fail.

  git fetch ==> fails:

  fatal: Invalid path '/home/somedir/worktree-tests': No such file or directory

  git status ==> fails -- same error as above
  git help worktree ==> fails -- same error as above