Visual diff: communicate what's the base/target version

[humitos]

Currently, the user is not aware what are the base/target versions diff'ed when looking at the visual diff UI element.

I got this feedback from a user in a support request:

While we're at it, and it's not critical or anything but it would be useful to convey to the reader of the diffed pages, what's the source and target branches for the said diff

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