patch 9.1.0172: More code can use ml_get_buf_len() instead of STRLEN()

Problem:  More code can use ml_get_buf_len() instead of STRLEN().
Solution: Change more STRLEN() calls to ml_get_buf_len().  Also do not
          set ml_line_textlen in ml_replace_len() if "has_props" is set,
          because "len_arg" also includes the size of text properties in
          that case. (zeertzjq)

closes: #14183

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/diff.c b/src/diff.c
index d2089dc..e1c8f58 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -762,7 +762,7 @@
 
     // xdiff requires one big block of memory with all the text.
     for (lnum = 1; lnum <= buf->b_ml.ml_line_count; ++lnum)
-	len += (long)STRLEN(ml_get_buf(buf, lnum, FALSE)) + 1;
+	len += ml_get_buf_len(buf, lnum) + 1;
     ptr = alloc(len);
     if (ptr == NULL)
     {