fix(config): skip deprecation warning when integrations key is present

Problem: `migrate_integrations` warned about old top-level keys even
when the user had already adopted `integrations`, which would be noisy
for configs that happen to have both.

Solution: only emit `vim.deprecate` and migrate values when
`opts.integrations` was not explicitly provided. When it is, old
top-level keys are silently stripped.

lua/diffs/init.lua

@@ -604,16 +604,17 @@ end
 local integration_keys = { 'fugitive', 'neogit', 'gitsigns', 'committia', 'telescope' }
 
 local function migrate_integrations(opts)
+  local has_new = opts.integrations ~= nil
   opts.integrations = opts.integrations or {}
   for _, key in ipairs(integration_keys) do
     if opts[key] ~= nil then
+      if not has_new then
         vim.deprecate(
           'vim.g.diffs.' .. key,
           'vim.g.diffs.integrations.' .. key,
           '0.3.2',
           'diffs.nvim'
         )
-      if opts.integrations[key] == nil then
         opts.integrations[key] = opts[key]
       end
       opts[key] = nil