filter-repo: fix cache of file renames

Users may have long lists of --path, --path-rename, --path-regex, etc.
flags (or even a --paths-from-file option with a lot of entries in the
file).  In such cases, we may have to compare any given path against a
lot of different values.  In order to avoid having to repeat that long
list of comparisons every time a given path is updated, we long ago
added a cache of the renames so that we can compute the new name for a
path once and then just reuse it each time a new commit updates the old
filepath.

Sadly, I flubbed the implementation and instead of setting
   cache[oldname] = newname
I somehow did the boneheaded
   cache[newname] = newname
For most repositories and rewrites, this would just have the effect of
making the cache useless, but it could wreak various kinds of havoc if
a newname matched the oldname of some other file.

Make sure we record the mapping from OLDNAME to newname to fix these
issues.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -3254,11 +3254,12 @@ class RepoFilter(object):
       if change.filename in self._newnames:
         change.filename = self._newnames[change.filename]
       else:
+        original_filename = change.filename
         change.filename = newname(args.path_changes, change.filename,
                                   args.use_base_name, args.inclusive)
         if self._filename_callback:
           change.filename = self._filename_callback(change.filename)
-        self._newnames[change.filename] = change.filename
+        self._newnames[original_filename] = change.filename
       if not change.filename:
         continue # Filtering criteria excluded this file; move on to next one
       if change.filename in new_file_changes: