perf(citation): optimize CITATION.cff rendering (#39575)

Rendering a CITATION.cff could use memory far out of proportion to the
file, as every YAML alias copies its target into the formatted citation
and the parser copies `%TAG` prefixes into every node. Files past these
limits show no citation, like unparseable ones do today.

- Skip files over 256 KiB, largest real-world file found is 80 KiB
- Skip files with `%TAG` directives
- Skip files whose aliases add more than 64 Ki nodes and value bytes
- Skip self-referencing anchors, except a sequence listing itself

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/git/tree_entry.go

@@ -124,6 +124,9 @@ func EntryFollowLinks(ctx context.Context, gitRepo *Repository, commit *Commit,
 	if treeEntry.IsLink() {
 		return res, util.ErrorWrap(util.ErrUnprocessableContent, "%q has too many links", firstFullPath)
 	}
+	if res == nil {
+		res = &EntryFollowResult{TargetEntry: treeEntry, TargetFullPath: fullPath} // in case limit=0
+	}
 	return res, nil
 }