Severity: wishlist
Tags: patch

Currently diff-goto-source with no prefix arg jumps to the new file,
and with a prefix arg jumps to the old file.

It does nothing special when the diff buffer is created by
a version control system, because the same current file is
visited by old and new.  This is a useful default behavior
to visit the current file and continue making changes
at an approximate location of the difference.

However, often this fails to find the change in old revisions.

So the proposed feature is backward-compatible: in the diff buffer
created by a version control system, with a prefix arg it will jump
to an old revision of the file.

It is very useful to look at the context of changed lines 
of the file as it was at the time of that old revision.