update multipolygon on old-style multipolygon action cannot be fully undone

[GerdP]

What steps will reproduce the problem?

1. Load attached file with an old style multipolygon
2. Select the way and press Ctrl+Shift+B to update the multipolygon
3. Undo

What is the expected result?

no changes in way or relation

What happens instead?

relation has tag building=yes (as well as the building)

Please provide any additional information below. Attach a screenshot if possible.

Build-Date:2020-12-13 12:15:19
Revision:17405
Is-Local-Build:true

Identification: JOSM/1.5 (17405 SVN en) Windows 10 64-Bit
OS Build number: Windows 10 Home 2004 (19041)
Memory Usage: 796 MB / 1820 MB (438 MB allocated, but free)
Java version: 1.8.0_272-b10, AdoptOpenJDK, OpenJDK 64-Bit Server VM
Look and Feel: com.sun.java.swing.plaf.windows.WindowsLookAndFeel
Screen: \Display0 1920×1080 (scaling 1.00×1.00)
Maximum Screen Size: 1920×1080
Best cursor sizes: 16×16→32×32, 32×32→32×32
VM arguments: [-agentlib:jdwp=transport=dt_socket,suspend=y,address=localhost:60338, -ea, -javaagent:D:\eclipse-java-2020-09\eclipse\configuration\org.eclipse.osgi\215\0\.cp\lib\javaagent-shaded.jar, -Dfile.encoding=UTF-8]

Plugins:
+ ColumbusCSV (35640)
+ buildings_tools (35669)
+ o5m (35640)
+ pbf (35650)
+ poly (35640)
+ reltoolbox (35640)
+ reverter (35640)
+ undelete (35640)
+ utilsplugin2 (35671)

Validator rules:
+ c:\josm\core\resources\data\validator\geometry.mapcss

[GerdP]

regression of r15160, since then no change command for the relation was created

[GerdP]

In 17408/josm:

fix #20238: update multipolygon on old-style multipolygon action cannot be fully undone

• make sure that a command to update the relation is generated
• add unit test