text: upon Enter, eat only lefthand blanks, not any other characters

Make sure that there is only whitespace to the left of the cursor
before setting 'allblanks' to TRUE, because this latter value will
cause these characters to be eaten (as a special case, to avoid
creating lines that contain only blanks when both --autoindent
and --breaklonglines are on).

This fixes https://savannah.gnu.org/bugs/?63407.
Reported-by: Tasos Papastylianou <tpapastylianou@hotmail.com>

Bug existed since version 2.9.8, commit d00ab406.

src/text.c

@@ -877,7 +877,7 @@ void do_enter(void)
 		if (extra > openfile->current_x)
 			extra = openfile->current_x;
 		else if (extra == openfile->current_x)
-			allblanks = TRUE;
+			allblanks = (indent_length(openfile->current->data) == extra);
 	}
 #endif /* NANO_TINY */
 	newnode->data = nmalloc(strlen(openfile->current->data +