Fix: Ensure winId is carried over to the pushed context

[kyoh86]

Issue

When calling ddu.vim's start function with push=true, the context.winId is incorrectly set to the winId of an already open ddu.vim instance. This results in the return window selection being incorrect after closing the ddu.vim window.

Fix

To address this issue, the winId is now carried over from the previous ddu context when push=true. This ensures that the context.winId remains consistent with the previously opened ddu instance, allowing the return window to be selected correctly.

Verification

Tests and verifications have been conducted to ensure that the winId is correctly carried over and that the return window selection works as expected.

:new
:wincmd j
:call ddu#start(...)
:call ddu#ui#do_action("chooseAction")
:q
:q

[Shougo]

OK. Merged.