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 suggesting to remove the call to this function, this parameter is hardcoded (plain string) so can't it be trusted? What do you think?

BTW: thx for this great tool!