fix(cwd): E812 when a message opens a window during a file read #41458

Problem:
When ui2 is enabled, opening an already-open file in another Neovim
instance results in the error `E812: Autocommands changed buffer…`.

Analysis:
On E812 the file is not loaded.  The default SwapExists handler notifies
`W325: Ignoring swapfile…`, and ui2 shows that message by opening
a window, which is a temp context switch.  `ctx_dirs_restore()`
re-shortens every buffer name on the way out, and `shorten_buf_fname()`
always frees and reallocates `b_sfname`.  `readfile()` aliases `b_fname`
across `check_need_swap()` and compares the pointer to detect a rename.

Regression by b296666e41, which replaced the `cs_save_sfname` restore
(that kept curbuf's pointer) with `shorten_fnames(true)`.

Solution:
Keep the allocation in `shorten_buf_fname()` when the short name is
unchanged.  Pointer stability is what the E200/E201/E812 guards actually
assert.

test/functional/ex_cmds/swapfile_preserve_recover_spec.lua

@@ -436,6 +436,46 @@ describe('swapfile detection', function()
     nvim1:close()
   end)
 
+  -- ui2 opens windows to show a message, which chdirs in and out of the target window's context.
+  -- readfile() aliases the buffer name across that.
+  it('ui2 message from the SwapExists handler does not abort the read #41454', function()
+    exec(init)
+    write_file('Xfile1', 'some text...\n')
+    command('edit Xfile1')
+    command('preserve') -- Make sure the swap file exists.
+
+    local initfile = 'Xtest_swapdialog_init.lua'
+    write_file(
+      initfile,
+      ('vim.opt.directory:prepend(%q)\nrequire("vim._core.ui2").enable()\n'):format(swapdir .. '//')
+    )
+    finally(function()
+      os.remove(initfile)
+      os.remove('Xfile1')
+    end)
+
+    local nvim1 = n.new_session(true)
+    set_session(nvim1)
+    local screen = Screen.new(75, 10)
+    -- ui2 needs a UI attached while the file argument is read, i.e. a real TTY.
+    fn.jobstart({ nvim_prog, '-i', 'NONE', '--noplugin', '-u', initfile, 'Xfile1' }, {
+      term = true,
+      env = { VIMRUNTIME = os.getenv('VIMRUNTIME') },
+    })
+    screen:expect({
+      grid = [[
+        ^some text...                                                               |
+        ~                                                                          |*6
+        Xfile1                                                   1,1            All|
+        {MATCH:W325: Ignoring swapfile from Nvim process %d+ *}|
+                                                                                   |
+      ]],
+      attr_ids = {},
+    })
+
+    nvim1:close()
+  end)
+
   it('attention message kind', function()
     exec(init)
     command('edit Xfile1')