Gogs: Allow "Rebase and merge" in pull requests

[InExtremaRes]

Description

At least at version 0.11.29.0727 a pull request can only be merged as a git merge --no-ff command. GitHub, for instance, allows _Squash and merge_ and _Rebase and merge_. Rebase and merge are very useful when project teams seek a clean, linear history.

With this feature a _topic branch_ should be rebased on top of the _default branch_ and then merged using the fast forward option.

More info about GitHub rebase and merge

[unknwon]

It is now possible on develop branch, you can also test on https://try.gogs.io.

Settings:

Merge options: