Fix double character in non-git dir (#5)

This commit fixes an issue where a the first character would be repeated
in the following scenario:

1. cd to directory that's not part of a git repo
2. Type 'a'
3. Hit tab
4. Cancel with ctrl-c
5. Double aa shows up

The fix is to print nothing at all if there is no output.

cmd/gitprompt/gitprompt.go

@@ -115,6 +115,9 @@ func main() {
 		_, _ = fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)
 	}
+	if s == nil {
+		return
+	}
 	out, num := gitprompt.Print(s, format.String())
 	_, _ = fmt.Fprint(os.Stdout, out)
 	if *zsh {