When looking at a patch in diff-mode, one can visit the sources being
patched with several functions, for instance (diff-goto-source). This
patch makes sure that the source buffer does not cover up the diff-mode
buffer in its window, but uses any other window instead.

This really is a continuation of bug 20034 (and 17675 and 19901). I feel
like the earlier display-buffer implementation had a nicer default in
this regard. Should we keep adding this patch wherever a user operation
on a buffer can cause a different buffer to be displayed, or should
display-buffer defaults be changed to make the "don't steal user's
focus" behavior the default?