[Proposal] add option to update pull request by ``rebase``

[a1012112796]

As we all know, do git rebase main(base_branch) and then force push to the head branch also can set the base of head branch update to the base branch, So I think maybe we can also add it as
a optional way on ui. Thanks

[lunny]

Didn't we have implemented that? You can find a button named Update when the pull request base head branch moved.

[nils91]

The update button does a git merge base_branch to update the PR, not a rebase. I think the relevant function is in
services/pull/update.go.

[a1012112796]

@lunny I know it, It use git merge base_branch, but as we all know, git rebase base_branch can do same thing. So like we provide more than one way to merge a pull request. I think we can also provide more than one way to update pull request also, Thanks.