When generating a stream clone, we spend a large amount of time auditing path.
Before this changes, the first run was warming the vfs cache for the other
runs, leading to a large runtime difference and a "faulty" reported timing for
the operation.

We now clear this important cache between run to get a more realistic timing.

Below are some example of median time change when clearing these cases. The
maximum time for a run did not changed significantly.

### data-env-vars.name             = mozilla-central-2018-08-01-zstd-sparse-revlog
  # benchmark.name                = hg.perf.exchange.stream.generate
  # bin-env-vars.hg.flavor         = default
  # bin-env-vars.hg.py-re2-module  = default
  # benchmark.variants.version    = latest
no-clearing:    17.289905
cache-clearing: 21.587965  (+24.86%, +4.30)
 ## data-env-vars.name             = mozilla-central-2024-03-22-zstd-sparse-revlog
no-clearing:    32.670748
cache-clearing: 40.467095  (+23.86%, +7.80)
 ## data-env-vars.name             = mozilla-try-2019-02-18-zstd-sparse-revlog
no-clearing:    37.838858
cache-clearing: 46.072749  (+21.76%, +8.23)
 ## data-env-vars.name             = mozilla-unified-2024-03-22-zstd-sparse-revlog
no-clearing:    32.969395
cache-clearing: 39.646209  (+20.25%, +6.68)

In addition, this significantly reduce the timing difference between the
performance command, from the perf extensions and a `real `hg bundle` call
producing a stream bundle. Some significant differences remain especially on
the "mozilla-try" repositories, but they are now smaller.

Note that some of that difference will actually not be
attributable to the stream generation (like maybe phases or branch map
computation).

Below are some benchmarks done on a currently draft changeset fixing some
unrelated slowness in `hg bundle` (34a78972af40)

### data-env-vars.name             = mozilla-central-2018-08-01-zstd-sparse-revlog
  # bin-env-vars.hg.flavor         = default
  # bin-env-vars.hg.py-re2-module  = default
hg.perf.exchange.stream.generate:  21.587965
hg.command.bundle:                 24.301799  (+12.57%, +2.71)
 ## data-env-vars.name             = mozilla-central-2024-03-22-zstd-sparse-revlog
hg.perf.exchange.stream.generate:  40.467095
hg.command.bundle:                 44.831317  (+10.78%, +4.36)
 ## data-env-vars.name             = mozilla-unified-2024-03-22-zstd-sparse-revlog
hg.perf.exchange.stream.generate:  39.646209
hg.command.bundle:                 45.395258  (+14.50%, +5.75)
 ## data-env-vars.name             = mozilla-try-2019-02-18-zstd-sparse-revlog
hg.perf.exchange.stream.generate:  46.072749
hg.command.bundle:                 55.882608  (+21.29%, +9.81)
 ## data-env-vars.name             = mozilla-try-2023-03-22-zlib-general-delta
hg.perf.exchange.stream.generate: 334.716708
hg.command.bundle:                377.856767  (+12.89%, +43.14)
 ## data-env-vars.name             = mozilla-try-2023-03-22-zstd-sparse-revlog
hg.perf.exchange.stream.generate: 302.972301
hg.command.bundle:                326.098755  (+7.63%, +23.13)