t5550: count fetches in "previously-fetched .idx" test

We have a test in t5550 that looks at index fetching over dumb http. It creates two branches, each of which is completely stored in its own pack, then fetches the branches independently. What should (and does) happen is that the first fetch grabs both .idx files and one .pack file, and then the fetch of the second branch re-uses the previously downloaded .idx files (fetching none) and grabs the now-required .pack file. Since the next few patches will be touching this area of the code, let's beef up the test a little by checking that we're downloading the expected items at each step. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com>