Bug: working directory is not reset after compile_tmux()

[Jensen Zhang]

After compile_tmux(), the working directory will be under $C9_DIR/tmux-1.9. It will make ptyjs() failed.

install.sh: 115

      # make sure dirs are around
      mkdir -p "$C9_DIR"/bin
      mkdir -p "$C9_DIR"/tmp
      mkdir -p "$C9_DIR"/node_modules
      cd "$C9_DIR"

      # install packages
      while [ $# -ne 0 ]
      do
        if [ "$1" == "tmux" ]; then
          time tmux_install $os $arch
          shift
          continue
        fi
        #===================================
        # somtimes, CWD is not $C9_DIR here
        #===================================
        time eval ${1} $os $arch
        shift
      done