> Users still can use all diff-mode commands to visit source files from them.
> Only language syntax fontification in diffs will fall back to hunk-only
> when a diff buffer is not created by a diff command.  This is to avoid all
> possible dangers of automatically visiting files in arbitrary diff buffers.

Due to the new variable `diff-default-directory', we can simplify the
customization options of `diff-font-lock-syntax'.  The default `t' is
the safest and the most reliable: it extracts revisions from VC when
the Diff buffer is created by a VC command, and extracts files from the
file system only in case the Diff buffer is created by a file-based
Diff command that sets the new variable `diff-default-directory'.

Here is a more less final version that I tested on many different diffs: