bug#28466 caused a regression in 26.0.50.  Maybe it's too late to fix it
in the release branch, so the patch below is for master.

The problem is that after the change a year and a half ago 
log-view-diff always falls back to the previous revision
even when point is in the middle of the log buffer,
and not after the last entry.

This patch uses the previous revision only at the end of the log buffer: