t/perf: add perf script for partial clones

t/perf: add perf script for partial clones

We don't cover the partial clone feature at all in t/perf. Let's at
least run a few basic tests so that we'll notice any regressions.

We'll do a no-blob clone, and split it into two parts: the actual object
transfer, and the subsequent checkout (which will of course require
another transfer to get the blobs). That will help us more clearly
assess the performance of each.

There are obviously a lot more possibilities besides just a no-blob
partial clone, but this should serve as a canary that alerts us to any
generic slow-downs (and we can add more tests later for cases that
aren't exercised here).

There are a few non-ideal things here that make this not an entirely
accurate test, but are probably OK for our purposes:

  1. We have to do some extra prep/cleanup work inside the timing tests,
     since they impact the on-disk state and the perf harness may run
     each one multiple times.

     In practice this is probably OK, since these bits should be much
     less expensive than the operations we are measuring.

  2. The clone time is likely to be dominated by the server's object
     enumeration. In the real world, a repo large enough to drive people
     to partial clones is likely to have reachability bitmaps enabled.

     And in the opposite direction, our object transfer is happening at
     the speed of a local pipe, whereas in the real world it would
     bottle-neck on the network.

     So any percentage speedups should be taken with a grain of salt.
     But hopefully any regressions will produce enough of an effect to
     be noticeable.

This script also demonstrates the recent improvement from dfa33a298d
(clone: do faster object check for partial clones, 2019-04-19):

  Test                          dfa33a298d^         dfa33a298d
  -------------------------------------------------------------------------
  5600.2: clone without blobs   18.41(22.72+1.09)   6.83(11.65+0.50) -62.9%
  5600.3: checkout of result    1.82(3.24+0.26)     1.84(3.24+0.26) +1.1%

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