Handle head sha changes correctly

[wilzbach]

It seems like we always try to merge with an old sha?

Response if sha was provided and pull request head did not match

Status: 409 Conflict
{
  "message": "Head branch was modified. Review and try the merge again.",
  "documentation_url": "https://developer.github.com/v3/pulls/#merge-a-pull-request-merge-button"
}

See also: dlang/phobos#5466 (comment)

[wilzbach]

Appeared here again: dlang/dmd#6760 (comment)

Not sure what's exactly causing this to appear only on a few PRs.

[wilzbach]

And here: dlang/dmd#7420

200 <190>1 2018-01-24T11:36:39.343284+00:00 app web.1 - - [048D4FF1:FD7CBDF0 ERR] PUT https://api.github.com/repos/dlang/dmd/pulls/7420/merge failed; Conflict 409.