test-lib.sh: preserve GIT_TRACE_CURL from the environment

Turning on this variable can be useful when debugging http
tests. It can break a few tests in t5541 if not set
to an absolute path but it is not a variable
that the user is likely to have enabled accidentally.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -89,6 +89,7 @@ unset VISUAL EMAIL LANGUAGE COLUMNS $("$PERL_PATH" -e '
 		UNZIP
 		PERF_
 		CURL_VERBOSE
+		TRACE_CURL
 	));
 	my @vars = grep(/^GIT_/ && !/^GIT_($ok)/o, @env);
 	print join("\n", @vars);