Quote project root in format string

Without this it's easy to cause an error if the directory name contains
shell syntax. E.g., in bash, try:

    $ cd ~/One (Two) && echo oops
    -bash: syntax error near unexpected token `('

To test the fix I invoked `helm-cmd-t` from a git repo whose path has a
left paren in it and no longer saw the above error reported in helm.