for #10634 Element duplication when copying root node

[xyin96]

for #10634
fixed element duplication when copy/pasting html tag.

The issue was sending a null as the oldElement in generateChildEdits, which implies an insertion edit.
The fix was to check if there was a root node in the oldNodeMap, and passing that node as the oldElement so that generateChildEdits had a node to compare to.

[humphd]

Fantastic. Glad to see a patch for this so quickly. @xyin96++

[AJDBenner]

Just tested this code and I'm getting an error in the console: "data-brackets-id=1 not found".

[xyin96]

Yeah this patch generates that error. I don't know how to fix it, but from what I've noticed, the referenced id is the tagID of the old html tag, which I assume has been replaced and deleted when the user copies and pastes an html document into the editor.
However, this error should not stop the live preview from updating correctly.

[busykai]

@xyin96, @humphd let me think about this and experiment a little bit. it's quite possible that this case (entire DOM re-instrumentation) should be handled by different means, not by editing existing DOM on the remote.