patch 9.1.1408: not easily possible to complete from register content

Problem:  not easily possible to complete from register content
Solution: add register-completion submode using i_CTRL-X_CTRL-R
          (glepnir)

closes: #17354

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/edit.c b/src/edit.c
index 4d45e8e..3661088 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -929,6 +929,8 @@
 	    break;
 
 	case Ctrl_R:	// insert the contents of a register
+	    if (ctrl_x_mode_register() && !ins_compl_active())
+		goto docomplete;
 	    ins_reg();
 	    auto_format(FALSE, TRUE);
 	    inserted_space = FALSE;