Review GitHub merge/rebase options

[cmungall]

I think this is responsible for some of the polluted histories

Currently when a commit is behind master we see this:

This associates the merge with the person merging polluting the branch history

E.g on #1449

[pbuttigieg]

What's the solution? Insisting on squash ?

[pbuttigieg]

@cmungall how do we need to modify the repo settings to fix this?

[cmungall]

I disallowed merge commits, our config is now the same as CL