Merge branch 'ep/use-git-trace-curl-in-tests' into maint

Update a few tests that used to use GIT_CURL_VERBOSE to use the
newer GIT_TRACE_CURL.

* ep/use-git-trace-curl-in-tests:
  t5551-http-fetch-smart.sh: use the GIT_TRACE_CURL environment var
  t5550-http-fetch-dumb.sh: use the GIT_TRACE_CURL environment var
  test-lib.sh: preserve GIT_TRACE_CURL from the environment
  t5541-http-push-smart.sh: use the GIT_TRACE_CURL environment var

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);