git-gui: default to full copy for linked worktrees

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>