fix(git): fix potential nil

lua/git/sidebar.lua

@@ -596,8 +596,6 @@ local function invocation_win_for(s)
     return win
 end
 
-local set_diff = diff.set_diff
-
 ---@param s ow.Git.SidebarState
 ---@param sidebar_win integer
 ---@return integer? left
@@ -720,8 +718,8 @@ local function show_diff(s, entry, focus_left)
     -- nvim_win_set_buf swaps the buffer pointer without invalidating
     -- cached diff state, and :diffupdate alone doesn't reliably force a
     -- recompute when no buffer contents have actually changed.
-    set_diff(left_win, false)
-    set_diff(right_win, false)
+    diff.set_diff(left_win, false)
+    diff.set_diff(right_win, false)
     vim.api.nvim_win_set_buf(left_win, pair.left.buf)
     vim.api.nvim_win_set_buf(right_win, pair.right.buf)
     for _, side in ipairs({ pair.left, pair.right }) do
@@ -738,8 +736,8 @@ local function show_diff(s, entry, focus_left)
     elseif right_ft == "" and left_ft ~= "" then
         vim.bo[pair.right.buf].filetype = left_ft
     end
-    set_diff(left_win, true)
-    set_diff(right_win, true)
+    diff.set_diff(left_win, true)
+    diff.set_diff(right_win, true)
     s.last_shown_key = key
 
     if focus_left then