Use GitHub-style commit message for squash merge

[Zettat123]

This PR modifies the commit message format for squash merge to match GitHub's style.

Screenshots

Before:

After:

GitHub:

[wxiaoguang]

Is it really a good result?

* Refactor user service
...
- Supports templating
* (others...)

Not sure whether GitHub does so. Even if GitHub does so, I don't think GitHub was right or we need to blindly follow GitHub

[Zettat123]

Sorry, I don't quite understand where the problem is. We add * before every commit message and - comes from the commit message content.

[wxiaoguang]

For example:

commit1:

a
* a
* c

commit2:

x
* y
* z

The current code generates:

* a
* b
* c
* x
* y
* z

It looks like there are 6 commits.

But ideally it should be:

* a
  * b
  * c
* x
  * y
  * z

Just my opinion.

[Zettat123]

Yes, GitHub does the same thing.

Using * in commit messages can make it difficult to distinguish between multiple messages. However, since the squash commit message is editable, I think that users can manually fix such issues if necessary.

[wxiaoguang]

What's the difference of " Squash merge messages contain information about all commits by default. #35247 " ? Why 35247 seems more complex and changed more code.

[Zettat123]

What's the difference of " Squash merge messages contain information about all commits by default. #35247 " ? Why 35247 seems more complex and changed more code.

Gitea already supports including the commit messages of all commits for squash merge in #16134 and this behavior is controlled by the POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES option. It seems that #35247 makes it the default behavior, and it is no longer controlled by the option. I think we should keep this option and only modify the format.