fix(clean-ignore): Consider .gitignore exclusions

I ran across what I think is the same issue as reported in #76.  My
repository's `.gitignore` has, among other things,

    *.mat
    !path/to/particular.mat

Running `clean-ignore --path-glob "*"` results in all `*.mat` files being
stripped from the repository history.  I'm using git version 2.39.3 (Apple
Git-146) and if I run

    git check-ignore --verbose --non-matching --no-index path/to/particular.mat

I see

    .gitignore:2:!path/to/particular.mat    path/to/particular.mat

It sounds like the expected behavior from `git check-ignore` is to see no
output, but that doesn't appear to be the case here.  In order to get the
`clean-ignore` script to correctly handle the exclusions in the `.gitignore`
file, I needed to make this slight tweak to it.

If it's the case that the real problem lies in `git` itself, and not in this
repository, then I could file an issue against it instead.

Signed-off-by: Jason M. Gates <jmgate@sandia.gov>

contrib/filter-repo-demos/clean-ignore

@@ -55,8 +55,12 @@ class CheckIgnores:
         if rest == b'::':
           self.okay.add(name)
         else:
-          self.ignored.add(name)
-          ignored.add(name)
+          rule = rest.split(b":")[-1]
+          if rule.decode()[0] == "!":
+            self.okay.add(name)
+          else:
+            self.ignored.add(name)
+            self.add(name)
 
     return ignored