In other words, this cherry-picking can generally be scripted and done
"outside" the SCM (you can trivially have a script that takes a revision
from one tree and applies it to the other). I don't believe that the SCM
needs to support it in any fundamentally inherent manner. After all, why should it, when it really boilds down to
(cd old-tree ; scm export-as-patch-plus-comments) |
(cd new-tree ; scm import-patch-plus-comments)
where the "patch-plus-comments" part is just basically an extended patch
(including rename information etc, not just the comments).