patch 8.0.0512: check for available characters takes too long

Problem:    Check for available characters takes too long.
Solution:   Only check did_start_blocking if wtime is negative. (Daisuke
            Suzuki, closes #1591)

src/os_unix.c

@@ -511,7 +511,7 @@ mch_inchar(
 		|| interrupted
 #endif
 		|| wait_time > 0
-		|| !did_start_blocking)
+		|| (wtime < 0 && !did_start_blocking))
 	    continue;
 
 	/* no character available or interrupted */

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    512,
 /**/
     511,
 /**/