bug#61215: 29.0.60; font-lock broken in diff-mode with long lines

[Gregory Heytings]

> Actually, these files can't expose the problem. I suggest to use a mode 
> with complex font-lock rules for syntax highlighting.  I tested with a 
> few of 1-char edits in a few places inside an .el file.

Okay, so I tried again with:

sed -i 's/(signal /(signal  /' lisp/subr.el lisp/simple.el

(Note the two spaces after "signal".)

> > git add a.xml
>
> The diff will output the file name a.xml at the beginning, but better to 
> output it after the file with changes.  The complete diff output should 
> look like this: first a few diff hunks each with 1-line change from an 
> .el file with syntax fontification. Then at the end of the screen the 
> huge file added in the same commit.

With my previous recipe the added file was added at the end, but for some 
reason after the command above it does not.  I renamed the a.xml file into 
z.xml, and now it does again.

> > git commit -a -m commit
> > ./src/emacs -Q --eval '(setq debug-on-error t backtrace-on-redisplay-error t)'
> > C-x v l
> > d
>
> Not sure if this shows diffs only from one file. More reliable would be 
> to use 'C-x v L d' to show a multi-file commit.

Okay, with the modifications you indicated above, I can now reproduce the 
bug, with both C-x v l d and C-x v L RET d.