Don't block for child exit if other commands have completed

Don't block for child exit if other commands have completed

Running regression tests on a single CPU system shows that if a child
exits while we're checking prerequisites we might not notice it, and
then we might block waiting for the next child to exit before we try
to do more work even though we could.

* src/remake.c (update_goal_chain): Remember the command_count before
we start checking prerequisites and if it is different when we restart
at the top of the goal chain then don't block.