joh/careful silverfish (#153089)

* don't show `openChange` command for merge editor

https://github.com/microsoft/vscode/issues/151125

* disable and not-place the conflicting merge-conflict navigation commands for the merge editor

https://github.com/microsoft/vscode/issues/153087

* disable (configurable) merge-conflict featues (code lens, decorations) when git is configured to use merge editor

https://github.com/microsoft/vscode/issues/153087