trace: give repo_setup trace its own key

You no longer get this output with GIT_TRACE=1; instead, you
can do GIT_TRACE_SETUP=1.

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

t/t1510-repo-setup.sh

@@ -57,7 +57,7 @@ test_repo () {
 			export GIT_WORK_TREE
 		fi &&
 		rm -f trace &&
-		GIT_TRACE="$(pwd)/trace" git symbolic-ref HEAD >/dev/null &&
+		GIT_TRACE_SETUP="$(pwd)/trace" git symbolic-ref HEAD >/dev/null &&
 		grep '^setup: ' trace >result &&
 		test_cmp expected result
 	)