Completion should generate separate tasks for the root project.

[pmarschik]

Currently the completion for root project tasks are generated like compileJava etc. Invoking gradle compileJava invokes the compileJava on the root project AND all subprojects that have that task.

I propose to additionally generate completions for :compileJava etc. (i.e. prefixed with :) so the root project's task can be invoked specifically without triggering tasks on subprojects.