Select revisions to compare by picking rows

Two hidden fields hold the pair, picking a row pushes it as the target and
the previous target becomes the source, so any pair takes two picks. Source
and target carry the diff view's own colours, in the rows and in the sticky
bar's readout.

Rows are focusable and respond to Enter and Space, the readout is the
accessible equivalent of the colour, so it is aria-live.

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