Run GitGutter when re-entering split window.

Nearly identical to b6c5364, update the signs on the buffer when
re-entering the split window. This is probably only used in addition to
other Git related vim plugins, like vim-fugitive by @tpope, when used
inside the same terminal session. :Gcommit opens a new split window to
enter the message, and afterwards switches back to the window previously
in focus. This should automatically update the signs then.