dim: add additional prompts for pushing merges

It's not exactly trivial to add the smarts to properly check for pushing
backmerges, rebases, topic branches and subtree branches (such as
gvt). For a start, prompt the user with hints about what's going on.

Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
3 jobs for master in 57 seconds (queued for 8 seconds)