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>

environment.c

@@ -64,7 +64,7 @@ const char *excludes_file;
 enum auto_crlf auto_crlf = AUTO_CRLF_FALSE;
 enum eol core_eol = EOL_UNSET;
 int global_conv_flags_eol = CONV_EOL_RNDTRP_WARN;
-char *check_roundtrip_encoding = "SHIFT-JIS";
+const char *check_roundtrip_encoding = "SHIFT-JIS";
 enum branch_track git_branch_track = BRANCH_TRACK_REMOTE;
 enum rebase_setup_type autorebase = AUTOREBASE_NEVER;
 enum push_default_type push_default = PUSH_DEFAULT_UNSPECIFIED;