patch: fix --no-backup-if-mismatch regression

Problem reported by Sam James in:
https://lists.gnu.org/archive/html/bug-patch/2025-02/msg00014.html
https://bugs.gentoo.org/show_bug.cgi?id=949834
* src/patch.c (backup_if_mismatch_specified): New static var.
(get_some_switches): Set it.
(main): Default backup_if_mismatch only if not set on command line.
* tests/no-backup: New file.
* tests/Makefile.am (TESTS): Add it.

src/patch.c

@@ -118,6 +118,7 @@ static bool merge;
 static enum diff reject_format = NO_DIFF;  /* automatic */
 static bool make_backups;
 static bool backup_if_mismatch;
+static bool backup_if_mismatch_specified;
 static char const *version_control;
 static char const *version_control_context;
 static bool remove_empty_files;
@@ -196,7 +197,8 @@ main (int argc, char **argv)
     if (set_utc && setenv ("TZ", "UTC0", 1) < 0)
       pfatal ("setenv");
 
-    backup_if_mismatch = ! posixly_correct;
+    if (!backup_if_mismatch_specified)
+      backup_if_mismatch = !posixly_correct;
     if (make_backups | backup_if_mismatch)
       backup_type = get_version (version_control_context, version_control);
 
@@ -1050,9 +1052,11 @@ get_some_switches (int argc, char **argv)
 		usage (stdout, EXIT_SUCCESS);
 	    case CHAR_MAX + 5:
 		backup_if_mismatch = true;
+		backup_if_mismatch_specified = true;
 		break;
 	    case CHAR_MAX + 6:
 		backup_if_mismatch = false;
+		backup_if_mismatch_specified = true;
 		break;
 	    case CHAR_MAX + 7:
 		posixly_correct = true;

tests/Makefile.am

@@ -50,6 +50,7 @@ TESTS = \
 	mixed-patch-types \
 	munged-context-format \
 	need-filename \
+	no-backup \
 	no-mode-change-git-diff \
 	no-newline-triggers-assert \
 	preserve-c-function-names \

tests/no-backup

@@ -0,0 +1,56 @@
+# Copyright 2025 Free Software Foundation, Inc.
+#
+# Copying and distribution of this file, with or without modification,
+# in any medium, are permitted without royalty provided the copyright
+# notice and this notice are preserved.
+
+# Test the --no-backup-if-mismatch option
+
+. $srcdir/test-lib.sh
+
+require cat
+use_local_patch
+use_tmpdir
+
+# ==============================================================
+
+cat >my_file <<'EOF'
+/* ... */
+void baz();
+
+
+void baz() {
+	/* ... */
+}
+
+int main() {
+	int foo;
+	int bar;
+
+	/* ... */
+	baz();
+}
+EOF
+
+cat >my_file.patch <<'EOF'
+--- my_file	2025-02-16 11:22:12.881765792 +0000
++++ my_file_new	2025-02-16 11:22:12.881796732 +0000
+@@ -2,7 +2,7 @@
+ void baz();
+
+ void baz() {
+-	/* ... */
++	// ...
+ }
+
+ int main() {
+EOF
+
+unset POSIXLY_CORRECT
+
+check 'patch -N --no-backup-if-mismatch <my_file.patch || echo "Status: $?"' <<'EOF'
+patching file my_file
+Hunk #1 succeeded at 3 with fuzz 1 (offset 1 line).
+EOF
+
+ncheck 'test ! -f my_file.orig'