escaped manually the shell arg (fixes #93)

I've been able to dig into my issue (#93), the stack trace is pretty explicit, at some point it tries to execute ```git show -s --pretty=format:" H n an n ci n s n" "ac86c6368c8b1ed
27adb436af54d0398b64cab1f"```, the format is wrong, percents (%) are missing, it's because I'm on windows, and ```escapeshellargs()``` is replacing percents by spaces (https://github.com/fabpot/Sismo/blob/master/src/Sismo/Builder.php#L120).

I'm wondering if it is needed to call this function, the parameters are hardcoded so can't they be trusted?

I suggest removing them and simply putting double quotes around (at least for the %format%)? What do you think?