Pacify gcc 14 -Wanalyzer-infinite-loop

* gnulib.modules: Add stddef, for ‘unreachable’.
* src/compare.c (dumpdir_cmp): Tell GCC that the default case
is unreachable.  Make just one pass through the string,
instead of two passes (one via strcmp, another via strlen).

gnulib.modules

@@ -92,6 +92,7 @@ setenv
 snprintf
 stat-time
 stdbool
+stddef
 stdint
 stpcpy
 stdopen

src/compare.c

@@ -334,34 +334,37 @@ diff_special (void)
 static int
 dumpdir_cmp (const char *a, const char *b)
 {
-  size_t len;
-
   while (*a)
     switch (*a)
       {
       case 'Y':
       case 'N':
-	if (!strchr ("YN", *b))
+	/* If the null-terminated strings A and B are equal, other
+	   than possibly A's first byte being 'Y' where B's is 'N' or
+	   vice versa, advance A and B past the strings.
+	   Otherwise, return 1.  */
+	if (! (*b == 'Y' || *b == 'N'))
 	  return 1;
-	if (strcmp(a + 1, b + 1))
-	  return 1;
-	len = strlen (a) + 1;
-	a += len;
-	b += len;
-	break;
-
+	a++, b++;
+	FALLTHROUGH;
       case 'D':
-	if (strcmp(a, b))
+	/* If the null-terminated strings A and B are equal, advance A
+	   and B past them.  Otherwise, return 1.  */
+	while (*a)
+	  if (*a++ != *b++)
+	    return 1;
+	if (*b)
 	  return 1;
-	len = strlen (a) + 1;
-	a += len;
-	b += len;
+	a++, b++;
 	break;
 
       case 'R':
       case 'T':
       case 'X':
 	return *b;
+
+      default:
+	unreachable ();
       }
   return *b;
 }