Long overdue runtime update.

runtime/pack/dist/opt/termdebug/plugin/termdebug.vim

@@ -95,6 +95,7 @@ func s:StartDebug(cmd)
   let s:gdbwin = win_getid(winnr())
 
   " Connect gdb to the communication pty, using the GDB/MI interface
+  " If you get an error "undefined command" your GDB is too old.
   call term_sendkeys(gdbbuf, 'new-ui mi ' . commpty . "\r")
 
   " Sign used to highlight the line where the program has stopped.
@@ -299,6 +300,7 @@ func s:HandleCursor(msg)
 	  endif
 	endif
 	exe lnum
+	exe 'sign unplace ' . s:pc_id
 	exe 'sign place ' . s:pc_id . ' line=' . lnum . ' name=debugPC file=' . fname
 	setlocal signcolumn=yes
       endif