git-gui: default to full copy for linked worktrees
git-gui's default clone method is git-clone's default, and this uses hardlinks rather than copying the objects directory for local repositories. However, this method explicitly fails if a symlink (or .gitfile) exists in the path to the objects directory. Thus, the default clone option fails for worktrees created by git-new-workdir or git-worktree. git-gui's original do_clone trapped this error for a symlinked git-new-workdir tree, directly falling back to a full clone, while the updated git-gui using git-clone does not. (The old do_clone could not handle gitfile linked worktrees, however). Let's apply the more friendly fallback to a full clone in both these cases where git-clone behavior throws an error on the default method. Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>