Allow push on behalf of others for contributors

Allow contributors to push on behalf of others if they keep the author of one of the previous patchsets of the same change.

not yet tested

eclipse-cla/.settings/org.eclipse.jdt.core.prefs

eclipse.preferences.version=1
-org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.6
-org.eclipse.jdt.core.compiler.compliance=1.6
+org.eclipse.jdt.core.compiler.codegen.targetPlatform=1.8
+org.eclipse.jdt.core.compiler.compliance=1.8
org.eclipse.jdt.core.compiler.problem.forbiddenReference=warning
-org.eclipse.jdt.core.compiler.source=1.6
+org.eclipse.jdt.core.compiler.processAnnotations=disabled
+org.eclipse.jdt.core.compiler.release=disabled
+org.eclipse.jdt.core.compiler.source=1.8