t/perf: add perf tests for fetches from a bitmapped server

A server with bitmapped packs can serve a clone very
quickly. However, fetches are not necessarily made any
faster, because we spend a lot less time in object traversal
(which is what bitmaps help with) and more time finding
deltas (because we may have to throw out on-disk deltas if
the client does not have the base).

As a first step to making this faster, this patch introduces
a new perf script to measure fetches into a repo of various
ages from a fully-bitmapped server.

We separately measure the work done by the server (in
pack-objects) and that done by the client (in index-pack).
Furthermore, we measure the size of the resulting pack.

Breaking it down like this (instead of just doing a regular
"git fetch") lets us see how much each side benefits from
any changes. And since we know the pack size, if we estimate
the network speed, then one could calculate a complete
wall-clock time for the operation (though the script does
not do this automatically).

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

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