Make worktree rm actually work in the presence of submodules

Also adds the `--force` option to remove a worktree with unstaged
changes.

worktree/worktree.plugin.zsh

@@ -102,7 +102,22 @@ Manage git worktrees by branch name."
         fi
         local -a rm_args=()
         (( force )) && rm_args+=(--force)
-        if ! git worktree remove "${rm_args[@]}" "$wt_path"; then
+        local err
+        if err=$(git worktree remove "${rm_args[@]}" "$wt_path" 2>&1); then
+          :
+        elif [[ "$err" == *"working trees containing submodules"* ]]; then
+          if (( ! force )) && \
+             [[ -n "$(git -C "$wt_path" status --porcelain --ignore-submodules=all 2>/dev/null)" ]]; then
+            print -P "%F{red}error:%f worktree has uncommitted changes (use --force): $wt_path"
+            failed=1
+            continue
+          fi
+          if ! rm -rf "$wt_path" || ! git worktree prune; then
+            failed=1
+            continue
+          fi
+        else
+          print -- "$err" >&2
           failed=1
           continue
         fi