[Patch] Minor command class fixes

[michael2402]

I started to fix some minor command issues some time ago. I added test cases for them. Since this was very low priority I never took the time to finish it but it should be applied before doing too many Java8 changes.

Fixes include:

• ChangeNodesCommand fails if nodes are empty (to prevent a plugin from creating empty ways)
• ChangePropertyCommand did not count empty strings as not set
• ChangePropertyKeyCommand only sets modified if the object was changed
• ChangePropertyKeyCommand creates the right child commands
• ChangeRelationMemberRoleCommand: executeCommand() always returns true
• ChangeRelationMemberRoleCommand: threw Exception if executeCommand() did not change anything
• RotateCommand/ScaleCommand: Constructor has more flexible generics
• RotateCommand: Performance improvements
• SelectCommand: Copy the selection list
• *CommandTest: Increase test coverage to ~90% (real tests)

All patches in between should apply nicely. You can get them on github:
​https://github.com/michaelzangl/josm/commits/test-command (starting with Commands: tests )
​https://github.com/michaelzangl/josm/commits/test-command-2 (starting with Code cleanup.)

[Don-vip]

In 10663/josm:

fix #13223 - Minor command class fixes (patch by michael2402, modified) - gsoc-core