add git rebase command in a pull request GUI

Hi,

in our git flow process, in order to have a more readable git history, the developers in our team do use git rebase functionality frequently. Typically, the developers will do git pull --rebase on latest develop branch, and then open a PR. But sometimes after several days review or testing, when PR is allowed to be merged, it is somehow several commits before develop branch again. and this will require another rebase before the merge. A falled process will show as the yellow highlight commit merge as "bugfix/BSFB-478-clean-pc-lint-error" in attached screenshot.

we hope there would be more flexible by introducting a command button in the PR GUI, so that the PR merger could choose the way to merge: to rebase the feature branch on latest develop and then do the regular merge, or just use the regular merge.