patch 9.0.2039: completion shows current word after completion restart

Problem:  completion shows current word after completion restart
Solution: remove the word being completed after completion restart

The word being completed is shown in a completion list after completion
restart, because it isn't removed from the current buffer before
searching for matches.

Also adjust `Test_complete_add_onechar` to match the new behavior.

closes: #13349

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: nwounkn <nwounkn@gmail.com>
diff --git a/src/version.c b/src/version.c
index a0ac2d0..208d7a8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -705,6 +705,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2039,
+/**/
     2038,
 /**/
     2037,