patch 9.0.0214: splitting a line may duplicate virtual text

Problem:    Splitting a line may duplicate virtual text. (Ben Jackson)
Solution:   Don't duplicate a text property with virtual text. Make
            auto-indenting work better. (closes #10919)

src/vim.h

@@ -2778,6 +2778,7 @@ long elapsed(DWORD start_tick);
 // Flags for adjust_prop_columns()
 #define APC_SAVE_FOR_UNDO	1   // call u_savesub() before making changes
 #define APC_SUBSTITUTE		2   // text is replaced, not inserted
+#define APC_INDENT		4   // changing indent
 
 #define CLIP_ZINDEX 32000