Another problem exposed by maven-release-plugin: the JGit SCM implementation's check-in fails unless the Maven project is in the working copy root because it confuses the working copy's location with the Maven project's location.
The attached patch resolves the issue. Combined with the patch attached to
SCM-806, release:prepare now mostly succeeds with the JGit implementation. There is still a problem with the POM not being transformed correctly, but that's a problem in maven-release-plugin.