test-lib: preserve GIT_CURL_VERBOSE from the environment

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

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

t/test-lib.sh

@@ -91,6 +91,7 @@ unset VISUAL EMAIL LANGUAGE COLUMNS $("$PERL_PATH" -e '
 		VALGRIND
 		UNZIP
 		PERF_
+		CURL_VERBOSE
 	));
 	my @vars = grep(/^GIT_/ && !/^GIT_($ok)/o, @env);
 	print join("\n", @vars);