Pull request comments on unchanged lines should be expanded in the diff (BB-11695)

When rendering a diff, Bitbucket shows a few lines of context. It is possible to expand that context by clicking "...". A comment left for one of the "expanded" source code lines becomes invisible when the diff is later loaded by another user (I guess) or reloaded by the commenting user (tested).

The invisible comment is counted and preserved, but one has to expand the context again to actually see it, which makes that comment useless.