Allow editing commit messages when merging a stack

Generally loving all the new stack features, thank you! To make it more useful, it would be great if when merging a stack of PRs from the website if we were prompted to choose a commit message like we are if merging one PR at a time.

For further details, we require squash merges and I have a coworker who performs a ton of tiny commits per branch in the stack. The default commit message for the squash merges includes a separate line per commit + blank lines around each; this ends up as a LOT of vertical space in the standard `git log` output that is really unhelpful.

從合併一組 PRs 的網站流程開始，並將其與現有的單個 PR 合併提示進行比較。當 stack merge 支援可編輯的提交訊息（包括 squash merge），同時保留單個 PR 的目前行為，並避免 issue 中描述的冗長預設訊息時，這項工作就完成了。

由索引模型根據 Issue 內容生成。