t5551: make EXPENSIVE test cheaper

t5551: make EXPENSIVE test cheaper

We create 50,000 tags to check that we don't overflow the
command-line of fetch-pack. But by using run_with_cmdline_limit,
we can get the same effect with a much smaller number of
tags. This makes the test fast enough that we can drop the
EXPENSIVE prereq, which means people will actually run it.

It was not documented to do so, but this test was also the
only test of a clone-over-http that requires multiple POSTs
during the conversation. We can continue to test that by
dropping http.postbuffer to its minimum size, and checking
that we get two POSTs.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>