submodule: pass on http.extraheader config settings

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 allows us to configure these extra HTTP headers for use with
`git submodule update`, too. It requires somewhat special handling:
submodules do not share the parent project's config. It would be
incorrect to simply reuse that specific part of the parent's config.
Instead, the config option needs to be specified on the command-line or
in ~/.gitconfig or friends.

Example: git -c http.extraheader="Secret: Sauce" submodule update --init

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

builtin/submodule--helper.c

@@ -127,7 +127,9 @@ static int module_name(int argc, const char **argv, const char *prefix)
  */
 static int submodule_config_ok(const char *var)
 {
-	if (starts_with(var, "credential."))
+	if (starts_with(var, "credential.") ||
+			(starts_with(var, "http.") &&
+			 ends_with(var, ".extraheader")))
 		return 1;
 	return 0;
 }

t/t5551-http-fetch-smart.sh

@@ -286,7 +286,16 @@ test_expect_success 'custom http headers' '
 	test_must_fail git 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