code review prerequisites should be more prominent and detailed

[Martin Pool]

example: https://code.edge.launchpad.net/~gz/bzr/merge_directive_from_lines_iter/+merge/15344

If you're doing a code review and it depends on another branch, knowing about that dependency is important to being able to assess the review properly. However, at the moment the "prerequisite" header is in the yada yada headers that are normally not worth reading: the target (usually the development focus) and the diffstat.

If there is a prerequisite it would be good if that was more clear. And if there is an mp for the prerequisite, which seems likely, it would be good if that was linked and had some information about its status, as is done on the bugs page. Just doing this might fulfil the goal of making it more visually obvious.

[Michael Hudson-Doyle (mwhudson)]

Seems reasonable, though I don't have any great ideas how to do the UI myself.

[Colin Watson (cjwatson)]

One particular improvement we could make is illustrated by https://code.launchpad.net/~cjwatson/lazr.enum/py3/+merge/373718: in this case the target is at r32, the prerequisite is at r33, and the source is at r34, but the preview diff dropdown says "r34 into r32" which is misleading. In this case it happens to be identical to what "r34 into r33" would be; if the prerequisite isn't an ancestor of the target it might need to be "<source> into <target> after merging <prerequisite>" or similar.