ci: work around a problem with HTTP/2 vs libcurl v8.10.0 (#5165)

This patch implements a work-around, basically for the sake of Git for
Windows v2.46.2's CI build: The `macos-13` runner images still reference
libcurl v8.10.0, which has a known flaw that breaks t5559.30. Let's
detect this situation and skip that test case in that case.

While at it, also backport the work-around for a flaky test case
(t5512.40 "helper with refspec capability fails gracefully").

This fixes #5159.

t/t5551-http-fetch-smart.sh

@@ -403,7 +403,15 @@ test_expect_success CMDLINE_LIMIT \
 	)
 '
 
-test_expect_success 'large fetch-pack requests can be sent using chunked encoding' '
+# This is a temporary work-around for libcurl v8.10.0 on the macos-* runners;
+# see https://github.com/git-for-windows/git/issues/5159 for full details
+test_lazy_prereq UNBROKEN_HTTP2 '
+	test "$HTTP_PROTO" = HTTP/2 &&
+	test -z "$(brew info -q curl 2>/dev/null |
+		sed -n "/^Installed/{N;s/.*8\\.10\\.0.*/BROKEN HTTP2/p;}")"
+'
+
+test_expect_success UNBROKEN_HTTP2 'large fetch-pack requests can be sent using chunked encoding' '
 	GIT_TRACE_CURL=true git -c http.postbuffer=65536 \
 		clone --bare "$HTTPD_URL/smart/repo.git" split.git 2>err &&
 	{