In this case the default branch is inactive, therefore it is quite likely that
you want to develop in coolbranch (or merge with it now).
If you're sure you want to extend the default branch, just use more --force.

Still it would be nice if the push code would know more about named branches,
e.g. to notice that 'default' will grow a second head while some other named
branch will lose one.

I adjusted the title for this.