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>

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