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.