Keep merged PR info in hidden branch

[lafriks]

To make merged PR commit tab more stable and consistent for each PR there should be hidden branch created same as github does it - refs/pull/x/head

[lafriks]

I noticed that we do actually already keep such hidden branch, so it just needs to be reused in merged PR to display diff/commits from it

[lunny]

@lafriks and should that another PR or will be in #3188 ?

[lafriks]

@lunny that should be another PR to not make #3188 even larger