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)
diff --git a/src/textprop.c b/src/textprop.c
index 1df4840..d90d57f 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -1906,6 +1906,7 @@
  * Only for the current buffer.
  * "flags" can have:
  * APC_SUBSTITUTE:	Text is replaced, not inserted.
+ * APC_INDENT:		Text is inserted before virtual text prop
  */
     static adjustres_T
 adjust_prop(
@@ -1931,6 +1932,10 @@
     start_incl = (pt != NULL && (pt->pt_flags & PT_FLAG_INS_START_INCL))
 				|| (flags & APC_SUBSTITUTE)
 				|| (prop->tp_flags & TP_FLAG_CONT_PREV);
+    if (prop->tp_id < 0 && (flags & APC_INDENT))
+	// when inserting indent just before a character with virtual text
+	// shift the text property
+	start_incl = FALSE;
     end_incl = (pt != NULL && (pt->pt_flags & PT_FLAG_INS_END_INCL))
 				|| (prop->tp_flags & TP_FLAG_CONT_NEXT);
     // do not drop zero-width props if they later can increase in size
@@ -1982,6 +1987,7 @@
  * "flags" can have:
  * APC_SAVE_FOR_UNDO:	Call u_savesub() before making changes to the line.
  * APC_SUBSTITUTE:	Text is replaced, not inserted.
+ * APC_INDENT:		Text is inserted before virtual text prop
  * Caller is expected to check b_has_textprop and "bytes_added" being non-zero.
  * Returns TRUE when props were changed.
  */
@@ -2097,6 +2103,9 @@
 	cont_prev = prop.tp_col != MAXCOL && prop.tp_col + !start_incl <= kept;
 	cont_next = prop.tp_col != MAXCOL
 			   && skipped <= prop.tp_col + prop.tp_len - !end_incl;
+	// when a prop has text it is never copied
+	if (prop.tp_id < 0 && cont_next)
+	    cont_prev = FALSE;
 
 	if (cont_prev && ga_grow(&prevprop, 1) == OK)
 	{