git: use the proper filenode for deleted files in changelog.changelogrevision

Inline printing shows it still doesn't find the removed file in the test, but it
is at least a str instead of bytes like the immediate query before it that does
find files.  Looking at the database, it doesn't look like the remove was
recorded.  Additionally, `hg log -r 'removes("re:.*")'` stacktraces.

Differential Revision: https://phab.mercurial-scm.org/D10524

hgext/git/gitlog.py

@@ -238,7 +238,7 b' class changelog(baselog):'
             for r in self._db.execute(
                 'SELECT filename FROM changedfiles '
                 'WHERE node = ? and filenode = ?',
-                (hn, nullhex),
+                (hn, gitutil.nullgit),
             )
         ]
         c = self.gitrepo[hn]