Resolves #3655
We were seeing that when running helm upgrade with the reuse-values
flag enabled that you could end up in the position where overrides
a.k.a computed values from previous revisions were not being saved on
the updated revision. This left us in a weird position where some
computed values would disappear mysteriously in the abyss. That
happened because computed values from previous revisions weren't merged
with the new computed values every time the reuse-values flag was used.
This PR merges computed values from the previous revisions so you don't
end up in that kind of conundrum.

(cherry picked from commit 9266731d)