filter-repo: provide a changed-refs metadata file as well

For sensitive data removals, quickly determining which refs were changed
may be helpful (e.g. to see how many pull-requests/merge-requests/
change-reviews/whatever-you-want-to-call-it refs were modified).

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

Documentation/git-filter-repo.txt

@@ -444,6 +444,18 @@ references were (or were not) changed.
   * An all-zeros hash, or null SHA, represents a non-existent object.
     When in the "new" column, this means the ref was removed entirely.
 
+Changed References
+~~~~~~~~~~~~~~~~~~
+
+The `$GIT_DIR/filter-repo/changed-refs` file contains a list of refs that
+were changed.
+
+  * No header is provided
+  * Lists the subsets of refs from ref-map for which old != new
+  * While unnecessary since this provides no new information over ref-map,
+    it does make it easier to quickly determine which refs were changed by
+    the rewrite.
+
 First Changed Commits
 ~~~~~~~~~~~~~~~~~~~~~
 

git-filter-repo

@@ -4791,6 +4791,10 @@ class RepoFilter(object):
         if old_hash != new_hash:
           self._changed_refs.add(refname)
 
+    with open(os.path.join(metadata_dir, b'changed-refs'), 'bw') as f:
+      for refname in sorted(self._changed_refs):
+        f.write(b'%s\n' % refname)
+
     with open(os.path.join(metadata_dir, b'first-changed-commits'), 'bw') as f:
       for commit, undeleted_self_or_ancestor in sorted(first_changes.items()):
         f.write(b'%s %s\n' % (commit, undeleted_self_or_ancestor))