refactor(git): scratch buffers wipe by default, consolidate factories in util

lua/git/sidebar.lua

@@ -495,7 +495,7 @@ end
 ---@return ow.Git.DiffSide
 local function absent_pane(worktree, path, kind)
     return {
-        buf = diff.empty_buf(),
+        buf = util.empty_buf(),
         name = string.format(
             "[absent %s] %s",
             kind,
@@ -865,7 +865,7 @@ local function open(worktree)
     end
 
     local previous_win = vim.api.nvim_get_current_win()
-    local bufnr, win = util.new_scratch({ split = "left", bufhidden = "wipe" })
+    local bufnr, win = util.new_scratch({ split = "left" })
     vim.bo[bufnr].filetype = "gitsidebar"
 
     vim.wo[win].number = false