Error displaying result pane when :pwd contains spaces

If vim's current directory contains spaces, `:Dispatch` runs the command, but then fails to open the result pane, displaying the following error:

    Error detected while processing function dispatch#compile_command..dispatch#complete..<SNR>102_cgetfile:
    line   21:
    E172: Only one file name allowed: cd /home/sstokes/hello world
    Error detected while processing function dispatch#compile_command..dispatch#complete:
    line   12:
    E171: Missing :endif
    Error detected while processing function dispatch#compile_command:
    line   63:
    E171: Missing :endif

Steps to reproduce:

```bash
mkdir hello\ world
cd hello\ world
cat > Makefile <<MAKE
hello:
	echo Hello world
MAKE
vim Makefile
```

Then running `:Dispatch` or `:Make` will produce this error.