On some profiles (after disabling syntax and refined
fontification), this was taking >90% of the time to fontify a buffer.
* lisp/vc/diff-mode.el (diff--git-preamble-overlay)
(diff--git-footer-overlay): New variables.
(diff--git-preamble-end, diff--git-footer-start): Use them to cache
the result.
89b40650da