(Finsert_file_contents): Give up match-end only if

coding requires conversion.  Adjust end of non-matching text area
to multibyte character boundary if enable_multibyte_character is non-nil.

src/fileio.c

@@ -3291,7 +3291,8 @@ This does code conversion according to the value of\n\
 		 we cannot use this method; giveup and try the other.  */
 	      if (same_at_end > same_at_start
 		  && FETCH_BYTE (same_at_end - 1) >= 0200
-		  && ! NILP (current_buffer->enable_multibyte_characters))
+		  && ! NILP (current_buffer->enable_multibyte_characters)
+		  && CODING_REQUIRE_CONVERSION (&coding))
 		giveup_match_end = 1;
 	      break;
 	    }
@@ -3302,6 +3303,12 @@ This does code conversion according to the value of\n\
 	{
 	  /* We win!  We can handle REPLACE the optimized way.  */
 
+	  /* Extends the end of non-matching text area to multibyte
+             character boundary.  */
+	  if (! NILP (current_buffer->enable_multibyte_characters))
+	    while (same_at_end < ZV && ! CHAR_HEAD_P (POS_ADDR (same_at_end)))
+	      same_at_end++;
+
 	  /* Don't try to reuse the same piece of text twice.  */
 	  overlap = same_at_start - BEGV - (same_at_end + st.st_size - ZV);
 	  if (overlap > 0)