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.
Merge pull request #17 from chrisklaiber/master

Quote project root in format string