Easy way to verify gitignore files?

It's not hard to tell if we're missing a file that ought to be listed
in .gitignore --- git status will find that problem soon enough.
However, it seems that git isn't so willing to tell you about gitignore
patterns that cover too much, i.e. match files that are already in the
repository.  I found out by accident that you will only hear about this
when you try to "git add" such a file after changing it.  This seems
pretty dangerous, especially for people who are willing to rely on
"git commit -a" :-(

Is there any automated sanity check that we can run to find this sort
of problem?  I suspect that we probably have got some errors in the
.gitignore files, particularly in the back branches, and it would be
nice to find them now before they get in the way of normal development.
        regards, tom lane