bug#17522: diff-mode frustrates attempt to correct corrupted diff file.

[Alan Mackenzie]

Hi, Emacs.

On

GNU Emacs 24.4.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.8.7) of 
2014-04-19 on acm.acm

, I have a diff which has been corrupted during transmission by email.  I
have loaded this diff into a Diff Mode buffer.  At the head of one of the
hunks appears:

    @@ -3155,115 +3160,38 @

, and three times in the hunk a long line has got split somehow, like
this:

   -                 (message "Eval error in the `c-lang-defvar' or 
`c-lang-setvar' for
   `%s'%s: %S"

.  When I attempt to correct the patch by removing the spurious line feed
using M-^, the header line is (silently) corrupted by replacing the two
"number of lines" fields with zeros, thusly:

    @@ -3155,0 +3160,0 @

.  This is surely a bug.  The workaround seems to be to use Fundamental
Mode.

-- 
Alan Mackenzie (Nuremberg, Germany).