Fix fontification outside hunks in Git patches

Fix fontification outside hunks in Git patches

* lisp/vc/diff-mode.el (diff-font-lock-keywords): Don't fontify lines in
Git patches starting with + or - as added/removed, if they are either
before the first hunk, or in the email signature.  (Bug#75884)
(diff-buffer-type): Move definition up.
(diff--indicator-added-re, diff--indicator-removed-re): New variables.
(diff--git-preamble-end, diff--git-footer-start)
(diff--indicator-matcher-helper, diff--indicator-added-matcher)
(diff--indicator-removed-matcher): New functions.
* test/lisp/vc/diff-mode-tests.el (diff-mode-test-git-patch)
(diff-mode-test-git-patch/before-first-hunk)
(diff-mode-test-git-patch/signature): New tests.
* test/lisp/vc/diff-mode-resources/git.patch: New file.

(cherry picked from commit 10abb87f0519b3d53d6b7078703d3a0120e3aaa8)