bug#60660: Support git-formatted patches in diff-mode

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Sun, 08 Jan 2023 20:21:23 +0200
> 
> Currently, there is a lot of misfontification in diff-mode visiting a git 
> patch.
> Two dashes in the signature separator are highlighted with the diff-removed 
> face
> (the existing diff-prev-line-if-patch-separator is used for something else).
> Three dashes in the git patch header that separate a list of affected files
> are highlighted with the diff-hunk-header face, etc.
> 
> But the worst case that raises an error is when an exclamation mark is used
> at the beginning of the line in the git patch message.
> 
> This patch for emacs-29 fixes this bug, and the rest of fontification
> could be implemented in master.

OK for the emacs-29 branch (although I'm surprised that
diff-beginning-of-hunk can return a position that is not before
point).

Thanks.