extend gitAnnexLink special case for worktrees

extend gitAnnexLink special case for worktrees

Fix symlinks generated to annexed content when in adjusted unlocked branch in a
linked worktree on a filesystem not supporting symlinks.

Before this fix, the symlink generated by eg git-annex sync in an adjusted
unlocked branch contained ".git/worktrees/". There was a special case
for the similar problem with submodules, so just use it to handle this case as
well. To detect this case, rely on fixupUnusualRepos setting mainWorkTreePath.