Patch conflation shouldn't use patching to merge patches

[monopole]

Under kyaml, the patch conflict detector and merging mechanism
https://github.com/kubernetes-sigs/kustomize/blob/master/api/resmap/merginator.go#L76
uses a patching approach rather than an object merge approach.

I.e., to conflate N patches, the second patch is applied to the first patch,
and the third patch is applied to that, etc. instead of merging the fields without
treating them as patching each other.

This results in a problem when two or more patches are applied at the same level to
the same object. Not a frequent use case, but it can happen and we have a test
covering it in

api/krusty/multiplepatch_test.go: TestRemoveEmptyDirWithPatchesAtSameLevel

An attempt to use a patch to drop the volume emptyDir {} field by
patching it with emptyDir null works when used in a single patch, but
fails in a multi-patch context because the null field is dropped in the
patch conflation.

Possibly related: #2037

[monopole]

See https://github.com/kubernetes-sigs/kustomize/blob/master/api/krusty/multiplepatch_test.go#L322