bug#1422: diff upon trouble

[jidanni]

Digging deep into (info "(diff)Invoking diff") we learn
   An exit status of 0 means no differences were found, 1 means some
   differences were found, and 2 means trouble.
which should also be on diff's man page. Bug CC'd.
Also "trouble" should be explained.
P.S. maybe warn on diff a a: same file. OK never mind.

Now let's examine emacs M-x diff's final line on three different exit values:
0:Diff finished.  Tue Nov 25...
1:Diff finished (no differences).  Tue Nov 25...
2:Diff finished.  Tue Nov 25...

The latter should also mention *** Trouble Encountered *** even if it
is mentioned by the diff error messages. We want to see it on the
bottom line too.

No diff-version variable. Can't tell from diff.el either.
emacs-version "22.2.1". diff (GNU diffutils) 2.8.1