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.

2 files changed, 91 insertions, 0 deletions

diff --git a/test/lisp/vc/diff-mode-resources/git.patch b/test/lisp/vc/diff-mode-resources/git.patch
new file mode 100644
index 00000000000..05ec90d105c
--- /dev/null
+++ b/test/lisp/vc/diff-mode-resources/git.patch
@@ -0,0 +1,51 @@
+From 1234567890abcdef1234567890abcdef12345678 Mon Sep 17 00:00:00 2001
+From: Alyssa P. Hacker <alyssa.p.hacker@example.com>
+Date: Sun, 3 Mar 2025 10:30:00 -0400
+Subject: [PATCH] Subtle bug fixes and slight improvements
+
+- This is not a removed line
++ This is not an added line
+
+---
+ src/main.py | 10 +++++-----
+ 1 file changed, 5 insertions(+), 5 deletions(-)
+
+diff --git a/src/main.py b/src/main.py
+index 9f6c5fe43e47eab441232e54456c5c2b06297b65..7b3f91a8b4ed923c8f43183276e3ab36fe04f6c9 100644
+--- a/src/main.py
++++ b/src/main.py
+@@ -2,25 +2,24 @@
+
+ def main():
+     # Initialize the magic number generator
+-    magic_number = 42
+-    print("Magic number: ", magic_number)
+
+-    # TODO: Fix the infinite loop
+-    while True:
+-        print("This loop will never end")
++    magic_number = 73  # After reconsidering, 73 seems more appropriate
++    print("Updated magic number: ", magic_number)
+
++    # The infinite loop was probably not the best approach
++    # while True:
++    #     print("This loop will never end.")
+
+     # This part of the code handles other important tasks
+     print("Processing other tasks...")
+
+     # Error handling has been updated for clarity
+-    if not fixed_it_yet:
+-        print("ERROR: Still broken!")
++    if not fixed_it_yet:  # This should be fine now
++        print("ERROR: No longer an issue.")
+
+     # Exiting the function on a positive note
+-    print("Goodbye, cruel world!")
++    print("Goodbye, world!")
+
+ if __name__ == "__main__":
+     main()
+
+-- 
+2.40.0
diff --git a/test/lisp/vc/diff-mode-tests.el b/test/lisp/vc/diff-mode-tests.el
index cd3f613f532..bbd66824e48 100644
--- a/test/lisp/vc/diff-mode-tests.el
+++ b/test/lisp/vc/diff-mode-tests.el
@@ -557,5 +557,45 @@ baz"))))
 +1
 ")))))
 
+(ert-deftest diff-mode-test-git-patch ()
+  (let ((file (ert-resource-file "git.patch")))
+   (with-temp-buffer
+     (insert-file-contents file)
+     (diff-mode)
+     (font-lock-ensure)
+     (goto-char (point-min))
+     (re-search-forward "magic_number = 42")
+     (should (eq (get-text-property (match-beginning 0) 'face)
+                 'diff-removed))
+     (re-search-forward "magic_number = 73")
+     (should (eq (get-text-property (match-beginning 0) 'face)
+                 'diff-added)))))
+
+(ert-deftest diff-mode-test-git-patch/before-first-hunk ()
+  (let ((file (ert-resource-file "git.patch")))
+   (with-temp-buffer
+     (insert-file-contents file)
+     (diff-mode)
+     (font-lock-ensure)
+     (goto-char (point-min))
+     (re-search-forward "This is not a removed line")
+     (should (eq (get-text-property (match-beginning 0) 'face)
+                  'diff-context))
+     (re-search-forward "This is not an added line")
+     (font-lock-ensure)
+     (should (eq (get-text-property (match-beginning 0) 'face)
+                 'diff-context)))))
+
+(ert-deftest diff-mode-test-git-patch/signature ()
+  (let ((file (ert-resource-file "git.patch")))
+   (with-temp-buffer
+     (insert-file-contents file)
+     (diff-mode)
+     (font-lock-ensure)
+     (goto-char (point-max))
+     (re-search-backward "^-- $")
+     (should (eq (get-text-property (match-beginning 0) 'face)
+                  'diff-context)))))
+
 (provide 'diff-mode-tests)
 ;;; diff-mode-tests.el ends here