better Range:highlight(..) handling

spec/withbuf.lua

@@ -0,0 +1,10 @@
+local function withbuf(lines, f)
+  vim.opt_global.swapfile = false
+
+  vim.cmd.new()
+  vim.api.nvim_buf_set_lines(0, 0, -1, false, lines)
+  local ok, result = pcall(f)
+  vim.cmd.bdelete { bang = true }
+  if not ok then error(result) end
+end
+return withbuf