fix(init): prevent infinite filetype retry loop on did_filetype

Problem: the deferred filetype retry in ensure_cache created an infinite
invalidation loop. did_filetype() stays 1 for the lifetime of a buffer
opened via FileType autocmd — it never resets in vim.schedule callbacks.
Each retry reparsed, found has_nil_ft=true again, and scheduled another
retry. Each iteration cleared all extmarks via pending_clear, causing the
deferred syntax pass to bail on its tick check. Result: language syntax
highlighting never settled on buffers containing files with
function-handled extensions (.sh, .bash, .conf, etc).

Solution: add ft_retry_pending guard table. Set before scheduling the
retry, check before scheduling again. The flag is set while the callback
is pending and during the synchronous redraw inside it, preventing the
reparse from scheduling another iteration. Cleared after the callback
completes. Cleaned up on BufWipeout.

lua/diffs/init.lua

@@ -169,6 +169,9 @@ local fast_hl_opts = {} ---@type diffs.HunkOpts
 ---@type table<integer, boolean>
 local attached_buffers = {}
 
+---@type table<integer, boolean>
+local ft_retry_pending = {}
+
 ---@type table<integer, boolean>
 local diff_windows = {}
 
@@ -334,13 +337,15 @@ local function ensure_cache(bufnr)
       has_nil_ft = true
     end
   end
-  if has_nil_ft and vim.fn.did_filetype() ~= 0 then
+  if has_nil_ft and vim.fn.did_filetype() ~= 0 and not ft_retry_pending[bufnr] then
+    ft_retry_pending[bufnr] = true
     vim.schedule(function()
       if vim.api.nvim_buf_is_valid(bufnr) and hunk_cache[bufnr] then
         dbg('retrying filetype detection for buffer %d (was blocked by did_filetype)', bufnr)
         invalidate_cache(bufnr)
-        vim.cmd('redraw')
+        vim.cmd.redraw()
       end
+      ft_retry_pending[bufnr] = nil
     end)
   end
 end
@@ -864,6 +869,7 @@ function M.attach(bufnr)
     callback = function()
       attached_buffers[bufnr] = nil
       hunk_cache[bufnr] = nil
+      ft_retry_pending[bufnr] = nil
       if neogit_augroup then
         pcall(vim.api.nvim_del_augroup_by_id, neogit_augroup)
       end