t5548: refactor test cases by resetting upstream

t5548: refactor test cases by resetting upstream

Refactor the test cases with the following changes:

 - Calling setup_upstream() to reset upstream after running each test
   case.

 - Change the initial branch tips of the workspace to reduce the branch
   setup operations in the workspace.

 - Reduced the two steps of setting up and cleaning up the pre-receive
   hook by moving the operations into the corresponding test case,

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>