Respect merge strategy when updating a branch on the UI

[alwibrm]

Issue

Impacted version: 4.34.0

Deployment mode: standalone app

Problem description:
When opening a Pull Request it might happen, that the branch you want to merge from gets out of date. Therefore GitBucket offers the comfortable function to update the branch via Web UI. The merge strategy for this function seems to be "merge commit" always, no matter what you have defined as default merge strategy for the repository.

Since development teams normally commit themselves on a common mode of git usage it would be consistent when the merge strategy is also applied to the "update branch" function on the Web UI.

[Stemann311]

I would prefer to even have a completely detached configuration for this button, e.g. "update strategy". In our team, we use rebase to update our feature branches, but merge to get the feature branch into develop.