t5548: refactor to reuse setup_upstream() function

t5548: refactor to reuse setup_upstream() function

Refactor the function setup_upstream_and_workbench(), extracting
create_upstream_template() and setup_upstream() from it. The former is
used to create the upstream repository template, while the latter is
used to rebuild the upstream repository and will be reused in subsequent
commits.

To ensure that setup_upstream() works properly in both local and HTTP
protocols, the HTTP settings have been moved to the setup_upstream() and
setup_upstream_and_workbench() functions.

Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>