contrib/asv.conf.json

run-tests: cache hghave results

Spawning a process on Windows is expensive.  I've got a version of
test-lfs-test-server.t locally which prints the http request/responses that
totals 819 lines, with 149 conditional lines, 11 #if tests, and 2 test cases.
It takes just under 1 minute with this change to run both cases, vs just over
2 minutes without this change.  Worse, when I explored adding ui.debug to the
test, it takes 13 minutes due to all of the mismatches and retests, vs less than
1 minute with this change.  Overall, the difference when running all tests is
negligible- 103 minutes with this change, vs 105 without when using -j9.

It also looks like an exit value of 2 from `hghave` is treated specially, but
there's nothing preventing 2 missing features from also using this value.

{
    "version": 1,
    "project": "mercurial",
    "project_url": "https://mercurial-scm.org/",
    "repo": "..",
    "branches": ["default", "stable"],
    "environment_type": "virtualenv",
    "show_commit_url": "https://www.mercurial-scm.org/repo/hg/rev/",
    "benchmark_dir": "benchmarks",
    "env_dir": "../.asv/env",
    "results_dir": "../.asv/results",
    "html_dir": "../.asv/html"
}