fix(editor): shada "force" load clears v:oldfiles #41662

Problem:
`shada_read()` rebuilds `v:oldfiles` for any "forced" read, which first
*clears* the list. For an in-memory caller such as multicursor, the list
is never repopulated. So `v:oldfiles` is empty after `Q` + edit.

Solution:
Rebuild `v:oldfiles` only when `kShaDaGetOldfiles` is requested.
`shada_read_everything()` (`:rshada[!]`, startup) always requests it.

old bug from 411a06c8b6

test/functional/api/vim_spec.lua

@@ -2892,6 +2892,11 @@ describe('API', function()
       eq(1, eval('g:one'))
       eq('', eval('&shada'))
       eq(0, eval("get(g:, 'optionset', 0)"))
+
+      -- Does not touch v:oldfiles (only ":rshada!" rebuilds it).
+      command('let v:oldfiles = ["/a", "/b"]')
+      api.nvim_load_context(ctx)
+      eq({ '/a', '/b' }, eval('v:oldfiles'))
     end)
 
     it('errors when context dict is invalid', function()