filter-repo: mark another incompatibility with fast-export's -M and -C

I suspect at some point someone will try to pass -M or -C to
fast-export; may as well leave a note in the code about another place
that's incompatible while I'm thinking about it.

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

git-filter-repo

@@ -2919,7 +2919,7 @@ class RepoFilter(object):
 
     # Filter or rename the list of file changes
     orig_file_changes = set(commit.file_changes)
-    new_file_changes = {}
+    new_file_changes = {}  # Assumes no renames or copies, otherwise collisions
     for change in commit.file_changes:
       # NEEDSWORK: _If_ we ever want to pass `--full-tree` to fast-export and
       # parse that output, we'll need to modify this block; `--full-tree`