patch 9.1.1431: Hit-Enter Prompt when loading session files

Problem:  Hit-Enter Prompt when loading session files
Solution: use set+= for 'shortmess' to keep the existing flags
          (Miguel Barro)

closes: #17445

Signed-off-by: Miguel Barro <miguel.barro@live.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/session.c b/src/session.c
index 56d368a..6696e3c 100644
--- a/src/session.c
+++ b/src/session.c
@@ -687,9 +687,9 @@
 
     // set 'shortmess' for the following.  Add the 'A' flag if it was there
     if (put_line(fd, "if &shortmess =~ 'A'") == FAIL
-	    || put_line(fd, "  set shortmess=aoOA") == FAIL
+	    || put_line(fd, "  set shortmess+=aoOA") == FAIL
 	    || put_line(fd, "else") == FAIL
-	    || put_line(fd, "  set shortmess=aoO") == FAIL
+	    || put_line(fd, "  set shortmess+=aoO") == FAIL
 	    || put_line(fd, "endif") == FAIL)
 	goto fail;