lib/mp-readline: Make it easy to exit auto-indent mode by pressing enter.

This patch allows you to stop auto-indent by pressing enter on a second
blank line.  Easier than having to use backspace, and prevents new users
from getting stuck in auto-indent mode.

docs/reference/repl.rst

@@ -49,6 +49,10 @@ Finally type ``print(i)``, press RETURN, press BACKSPACE and press RETURN again:
     3
     >>>
 
+Auto-indent won't be applied if the previous two lines were all spaces.  This
+means that you can finish entering a compound statment by pressing RETURN
+twice, and then a third press will finish and execute.
+
 Auto-completion
 ---------------
 

lib/mp-readline/readline.c

@@ -370,6 +370,18 @@ STATIC void readline_auto_indent(void) {
             }
         }
         // i=start of line; j=first non-space
+        if (i > 0 && j + 1 == line->len) {
+            // previous line is not first line and is all spaces
+            for (size_t k = i - 1; k > 0; --k) {
+                if (line->buf[k - 1] == '\n') {
+                    // don't auto-indent if last 2 lines are all spaces
+                    return;
+                } else if (line->buf[k - 1] != ' ') {
+                    // 2nd previous line is not all spaces
+                    break;
+                }
+            }
+        }
         int n = (j - i) / 4;
         if (line->buf[line->len - 2] == ':') {
             n += 1;