Don't merge and perform timeline actions

[jablko]

Pedantic because:

1. It's unlikely that anyone would request to merge in the interval between the timeline expiring and the bot closing the PR
2. Because mergePullRequest comes before closePullRequest in getMutationsForChangingPRState(), I expect that the merge request would be honored anyway (closePullRequest would subsequently fail, and there'd be an extraneous Unmerged:done comment)

The fix is to only perform timeline actions if the PR isn't also about to be merged.

The follow-up commit combines shouldClose and shouldMerge, like #348, so it's obvious they're mutually exclusive.