Fix Dired display and operations on file names with raw bytes

* src/coding.c (decode_coding): When flushing remaining raw bytes
from multibyte text, copy 2-byte forms of eight-bit-*
characters as single characters, not as 2 raw bytes.  (Bug#29189)

src/coding.c

@@ -7423,10 +7423,23 @@ decode_coding (struct coding_system *coding)
 
 	  while (nbytes-- > 0)
 	    {
-	      int c = *src++;
+	      int c;
 
-	      if (c & 0x80)
-		c = BYTE8_TO_CHAR (c);
+	      /* Copy raw bytes in their 2-byte forms from multibyte
+		 text as single characters.  */
+	      if (coding->src_multibyte
+		  && CHAR_BYTE8_HEAD_P (*src) && nbytes > 0)
+		{
+		  c = STRING_CHAR_ADVANCE (src);
+		  nbytes--;
+		}
+	      else
+		{
+		  c = *src++;
+
+		  if (c & 0x80)
+		    c = BYTE8_TO_CHAR (c);
+		}
 	      coding->charbuf[coding->charbuf_used++] = c;
 	    }
 	  produce_chars (coding, Qnil, 1);