[Git worktrees](https://git-scm.com/docs/git-worktree) are supported since version 6.20180719.

Git normally makes a `.git` **file** in a
worktree, that points to the real git repository under `.git/worktrees/`.
This presents problems for git-annex. So, when used in a worktree,
git-annex will automatically replace the `.git` file with a symlink
pointing at the git repository. It also places an appropriate `annex` link
to `.git/worktrees/<name>/annex` to point to the object store. I don't know
how crippled filesystems are handled.

Getting, dropping and syncing content works fine in a worktree, however
if there is change in the tree then syncing doesn't update git worktrees
and their indices, but updates the checked out branches. This is different to
the handling of the main working directory as it's either got updated or left
behind with its branch if there is a conflict.

In its current state I use git-worktree to copy symlinks across branches and run `git annex fix`
on them. I only use temporary worktrees due to the syncing behavior.
