sequencer: simplify away extra git_config_string() call

In our config callback, we call git_config_string() to copy the incoming value string into a local string. But we don't modify or store that string; we just look at it and then free it. We can make the code simpler by just looking at the value passed into the callback. Note that we do need to check for NULL, which is the one bit of logic git_config_string() did for us. And I could even see an argument that we are abstracting any error-checking of the value behind the git_config_string() layer. But in practice no other callbacks behave this way; it is standard to check for NULL and then just look at the string directly. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>