>> +*** In the diff buffer created by a version control system, the prefix
>> +arg of diff-goto-source means it jumps to the old revision of the file
>                            ^^^^^^^^^^^^^^^^^
> "... means jump to ..." is a better wording, I think.
>
> Also, it is best to provide a header for the description of the
> change, so that it could be meaningfully folded by Outline mode.

Fixed in a new patch.

> And I think we want to update the manual as well.

Updated.

>> +(defvar diff-vc-revisions nil
>> +  "The VC revisions compared in the current Diff buffer, if any.")
>> +
>>  (defvar diff-outline-regexp
>>    "\\([*+][*+][*+] [^0-9]\\|@@ ...\\|\\*\\*\\* [0-9].\\|--- [0-9]..\\)")
>
> Which VCSes does this support?  I'm not sure all of them produce such
> markers.  If this supports only some, we should document that and make
> sure the code does something reasonable when the revision is not
> found.

It should work in all VCSes that support the VC interface function
'find-revision' defined in the comments of vc.el:

;; * find-revision (file rev buffer)
;;
;;   Fetch revision REV of file FILE and put it into BUFFER.
;;   If REV is the empty string, fetch the head of the trunk.
;;   The implementation should pass the value of vc-checkout-switches
;;   to the backend command.

BTW, its implementation in vc-find-revision currently has
a major shortcoming: it always saves the retrieved revision
to the file.  I spend too much time cleaning after this command
all old files that it creates.  Thus I propose a new option
`vc-find-revision-no-save' to not save the buffer it creates
to the file.