Way back when I still used TortoiseSVN, I really enjoyed the fact that it's commit comment textbox would suggest types and filenames when committing new changes. See the attached image for an example.

The simplest implementation would be for the client to check the pending changes, compile a list of the filepaths and filenames, and then determine if the current typed word would match one of these list entries. If a word partially matches one of the entries, show a dropdown of these entries to let the user choose or dismiss the entries.

Just to elaborate, this is not an auto-fill feature (like #6389015), but more an autocomplete used to aid in comments like "VirtualFileBrowser now caches previous opened items" or "Removed Delete method from AbstractObjectCollection".