Refactor preparePullViewPullInfo and related functions, split them into
small ones:
* preparePullViewPullInfo creates PullRequestViewInfo struct
* if the PR is merged: prepareView**Merged**PullInfo
* if the PR is open: prepareView**Open**PullInfo
In prepareViewMergedPullInfo and preparePullViewFillInfo: call
preparePullView**FillInfo** consistnently
preparePullViewFillInfo calls preparePullViewFill**CompareInfo** and
preparePullViewFill**CommitStatusInfo**