config: use git_config_string() for core.checkRoundTripEncoding

config: use git_config_string() for core.checkRoundTripEncoding

Since this code path was recently converted to check for a NULL value,
it now behaves exactly like git_config_string(). We can shorten the code
a bit by using that helper.

Note that git_config_string() takes a const pointer, but our storage
variable is non-const. We're better off making this "const", though,
since the default value points to a string literal (and thus it would be
an error if anybody tried to write to it).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>