The dlang bot should always try to rebase on ~master before merge

[Seb]

On Saturday, 4 May 2019 at 11:12:02 UTC, user1234 wrote:
> This would facilitate locating regressions or visual browsing 
> with gitk. If GH doesn't disable the merge button than there no 
> conflict anyway... Don't know if the GH API allows to detect 
> conflict.
>
> Other suggestion would be to squash automatacally if a 
> particular commit doesn't contain a reference to a distinct PR 
> (#<dec number>), git commit (<hex number>), or dlang issue (fix 
> issue <dec number>). Unrelated , unfocused changes are 
> discouraged anyway within a single PR.

Whenever there's a merge to master/stable, the Auto-Tester will 
invalidate all CI results and thus dlang-bot needs to wait again 
until all CI builds pass.

Also, the GitHub API wouldn't allow us to merge on a conflict.

Currently the bot doesn't do the merge itself, but uses the GH 
API, hence our options are a bit limited - though it shouldn't be 
too hard to do more actual work with the bot.

Here's the code for more details:

https://github.com/dlang/dlang-bot