Unecessary decision dialog while merging two ways that are part of a multipolygon

[naoliv]

Open the attached example, select the select=me parts and merge (with C).
We have this screen:

Note how JOSM asks if we want to keep both segments in the multipolygon.
Most of the time people will think "of course I want both segments" and mark them to be kept.

But if the user chooses this logical solution he will end with a broken multipolygon:

It's not clear that he needs to keep only one way (any of them) and remove the other to have the correct solution.

Since we are merging two ways into one, the decision dialog should ask us if we want to keep/remove the already merged way and not the 2 initial segments.

It's even worse when we have more segments involved in the merge process.

JOSM:

URL:http://josm.openstreetmap.de/svn/trunk
Repository:UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Last:Changed Date: 2016-07-13 23:20:15 +0200 (Wed, 13 Jul 2016)
Build-Date:2016-07-14 01:31:48
Revision:10531
Relative:URL: ^/trunk

Identification: JOSM/1.5 (10531 en) Linux Debian GNU/Linux testing (stretch)
Memory Usage: 247 MB / 10206 MB (58 MB allocated, but free)
Java version: 1.8.0_91-8u91-b14-3-b14, Oracle Corporation, OpenJDK 64-Bit Server VM
VM arguments: [-Dawt.useSystemAAFontSettings=on]
Dataset consistency test: No problems found

Plugins:
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