submodule: ensure that -c http.extraheader is heeded

To support this developer's use case of allowing build agents token-based
access to private repositories, we introduced the http.extraheader
feature, allowing extra HTTP headers to be sent along with every HTTP
request.

This patch verifies that we can configure these extra HTTP headers via the
command-line for use with `git submodule update`, too. Example: git -c
http.extraheader="Secret: Sauce" submodule update --init

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5551-http-fetch-smart.sh

@@ -287,7 +287,16 @@ test_expect_success 'custom http headers' '
 		fetch "$HTTPD_URL/smart_headers/repo.git" &&
 	git -c http.extraheader="x-magic-one: abra" \
 	    -c http.extraheader="x-magic-two: cadabra" \
-	    fetch "$HTTPD_URL/smart_headers/repo.git"
+	    fetch "$HTTPD_URL/smart_headers/repo.git" &&
+	git update-index --add --cacheinfo 160000,$(git rev-parse HEAD),sub &&
+	git config -f .gitmodules submodule.sub.path sub &&
+	git config -f .gitmodules submodule.sub.url \
+		"$HTTPD_URL/smart_headers/repo.git" &&
+	git submodule init sub &&
+	test_must_fail git submodule update sub &&
+	git -c http.extraheader="x-magic-one: abra" \
+	    -c http.extraheader="x-magic-two: cadabra" \
+		submodule update sub
 '
 
 stop_httpd