repo prune: Warn user when branch not merged
repo prune: Warn user when branch not merged

The command will now warn the user when they are about to delete a branch that
is not merged yet so that it is less probable that they delete a branch that is
supposed to stay there.

Change-Id: 3f56931cdc
Story-Id: salsaflow/salsaflow#102

salsaflow/salsaflow,salsaflow/salsaflow