patch 8.1.0685: get_buf_tv() is named inconsistently

Problem:    get_buf_tv() is named inconsistently.
Solution:   Rename it to tv_get_buf(). (Yegappan Lakshmanan, closes #3759)
diff --git a/src/terminal.c b/src/terminal.c
index d030f6d..9fb7fb9 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -3842,7 +3842,7 @@
 
     (void)tv_get_number(&argvars[0]);	    /* issue errmsg if type error */
     ++emsg_off;
-    buf = get_buf_tv(&argvars[0], FALSE);
+    buf = tv_get_buf(&argvars[0], FALSE);
     --emsg_off;
     if (buf == NULL || buf->b_term == NULL)
     {