* test/lisp/vc/diff-mode-tests.el (diff-mode-test-font-lock): New test.

* test/lisp/vc/diff-mode-tests.el (diff-mode-test-font-lock): New test.

(diff-mode-test-font-lock-syntax-one-line): New test for one line.

* test/data/vc/diff-mode/hello_world.c:
* test/data/vc/diff-mode/hello_world_1.c:
* test/data/vc/diff-mode/hello_emacs.c:
* test/data/vc/diff-mode/hello_emacs_1.c: New fixtures.

* lisp/vc/diff-mode.el (diff-syntax-fontify): Move remove-overlays
from diff-syntax-fontify-hunk.  (Bug#33567)
(diff-syntax-fontify-hunk): Remove VISIT arg from insert-file-contents.