t5538: move http push tests out to t5542

t5538: move http push tests out to t5542

As 0232852b, but for the push tests instead: this avoids a start_httpd
in the middle of the file, which fails under GIT_TEST_HTTPD=false.

Note that we have to munge the test in a few ways while
moving it:

  1. We drop the `test -z "$GIT_TEST_HTTPD"` check; this is
     too simplistic since 83d842d, and we should let
     lib-httpd.sh handle it.

  2. We have to port over some of the old setup from t5538.

  3. In the final test, we no longer expect the extra commit
     "1" built on top of "4". This was a side effect from an
     earlier test in t5538 which was not ported over.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>